I completely disagree with most of the other answers. I believe future chemical weapons will be larger and more powerful for a very simple reason: they will have to be.
Over the course of human history there has always been a "sword versus shield" balance that has tilted one way or another. The basic question: "does the standard weaponry of the day easily overcome the standard defenses most warriors will have?" is how you decide where you are on that spectrum. If technology has tilted one way and your enemy doesn't realize it yet, you can do some serious damage before he figures out what happened.
Examples: Thermopylae. Spartans knew that the standard defenses of the day (Hoplon/Greek style shield) easily overcame standard weapons of the day. They had a force totally optimized around a small group of elite warriors who are not easy to kill but which required significant money and time investment to field (hence smaller in number). The Persians had a massive army consisting of cheap cannon fodder including conscripted slaves. They were using the opposite paradigm. They "won" but at such a cost that it was a strategic loss.
The balance has shifted back and forth for centuries. In times when weapon lethality is dominant, large armies of cannon fodder are the way to go, in times when defenses are dominant, small armies of professional warriors dominate.
In medieval times, defensive technology had outstripped weapon lethality oncve again. Crusaders encountered armies many times their size and prevailed in many cases. Many accounts wrote of knights returning from battle covered in arrows like a porcupine, but with no major injuries. It was logistics that really stopped the crusaders of the earlier crusades, not technology or battles.
The pendulum came back with gunpowder. Armies organized around a small group of professional elites with good defensive technology were swept aside, but it took a while before someone really organized for optimum performance in the gunpowder era. If you look at Napoleon, it wasn't just that he was a good tactical commander, but what you have to understand is the revolutionary way that France had been totally transformed into a machine for churning out massive armies of cheap cannon fodder. Napoleon could crank out MULTIPLE 300,000+ man armies after burning through them one after another while his enemies struggled to keep one or two forces of that scale in the field. How? He basically created the modern nation state; a machine designed around the draft, which would efficiently crank out expendable soldiers capable of standing in a line and using very simple weapons, marching, and dying on command.
I won't go through WWI, WWII, The American Civil War, etc, but we have CLEARLY been in a "lethality" paradigm for a while now. WWII was almost completely a war of attrition.
What is interesting is that today we are undergoing a revolution in materials technology. Ceramic plates capable of stopping a machine gun round are normal for our military. That is totally revolutionary, and it turns Napoleon's paradigm on it's head. We can now make a warrior totally bulletproof. The only limitation is the weight of the armor. That puts us back somewhere between the Spartans and the European knights, which means that smaller, more professional, more skilled, and more expensive militaries will prevail (and we see this trend increasing, just compare WWII, Vietnam, Desert Storm, and the recent war in Iraq).
Soon, the dominant paradigm will be VERY elite and professional lifelong warriors wearing 100% complete bulletproof armor suits and operating very complicated and high tech weapons, at a very high cost per warrior (so there will be few of them). In this environment chemical weapons will have to get more lethal (as melee weapons did all through the middle ages as engineers sought to penetrate ever better armor).
How? Given the limitations of chemical propellants already mentioned, think about some options:
1 If we solve the "armor weight" issue using some sort of robotic exoskeleton (as the US Army is currently working on) we have also allowed an infantryman to carry something closer to a .50 cal machine gun. We have also created a need for him to have more firepower since his opponents will probably be "bulletproof" to a typical light round like the .225 currently considered NATO standard.
2 Chemical propellants might have maximum expansion velocities, but we have already figured out how to optimize the technology in a few ways that aren't cost effective for a "cannon fodder" army but will be used in the new paradigm era. DU rounds drastically increase the mass of the bullet and thus it's ability to carry kinetic energy to target. As a side benefit, they also create burning gas sometimes, which increases lethality.
3 Discarding sabot ammunition minimizes the drag on a projectile while maximizing the power of your propellant. This is the direction "infantry" weapons will probably take as warriors have to overcome one another's ever better armor in years to come.
4 Rocket assisted ammunition can continue to accelerate after it has left the weapon. Advanced designs include the ability to mix and match rocket assisted and "conventional" ammunition in the same weapon.
5 Advanced "supergun" concepts designed to launch satellites have experimented with multiple, sequential firing chambers. Chamber one cooks off, bullet moves down the barrel, at a certain point, chamber 2 (halfway down the barrel) cooks off, and bullet gets even more energy. This could be miniaturized.
6 Explosive payloads can be integrated into larger caliber infantry weapons as a standard feature (once again, to overcome armor). Think miniaturized HEAT rounds (High Explosive Anti Tank) that form a plasma "jet" when the round comes into proximity with the target's armor.
7 Some nice new concepts like "metal storm" -electrically initiated conventional chemical projectiles and helical ammunition storage promise to radically increase the firing rate and ammunition storage possibilities of small arms.
Most of this stuff is already being done at the scale of vehicle weapons, tanks, etc. The difference is; as infantry get more armor, they are also going to start adapting these sorts of tactics to OVERCOME that same armor. This is already happening today. Unless there is a major technological sea change, expect the future to be more "starship troopers" (the book, not the idiotic movie) and less "red badge of glory".