Polar bears might be the most extreme in the wild...but I get the feeling the breeders of the Chihuahua Husky mix might be a benchmark of the more extreme difference in partners size.
First to point out - size is one attribute that I can focus on, but it should be noted it's simply one attribute out of many. As a good example that was stated above, frogs are around the same size for a male and female, but (generally, some exceptions) only male frogs croak. Rather simple...the male frog is using croaking to attract a mate, which comes at the cost of attracting predators. Peacocks and Betta's use visual cue's with the males visual plume being it's method of attracting a mate. In these cases, the male is taking on the role of attracting the mate, often at a risk to it's own survival. The methodology of this 'attracting a mate' often becomes the basis of sexual dimorphism (this is an interesting line simply because it shows how evolution heavily contains a behavioral aspect)
Another point to consider...survival of the fittest/strongest isn't really true, on a genetic level it becomes more 'survival of those most capable of mating'. The fastest, strongest, smartest, fittest peacock around may never get the opportunity to breed if it lacks the plume. This is the point that really starts to define the differences between males and females in a species, those males possessing a trait that attracts them the mate and allows them to pass along their genetics will pass that attribute on to the next generation. This has the tendency to create some pretty extreme mating displays and some stark contrasts in female / male appearances and behavior.
If you want to highlight size between males and females in a species, then you need to focus on species that have the behavior that stress size as a mate attraction technique, usually in the form of strength. Antlered herbivores are good examples here...males fight between each other to win and defend their females. This process is usually the two stags sizing each other up before clashing antlers. However this doesn't necessarily promote sheer size...there are a lot of components of the fight such as the antlers, endurance, agility, etc...and although you will see a pronounced difference between males and females here, you won't find the biggest difference.
To get to the biggest difference, you need to find the mate attraction technique that values size above fighting skill. And you'll find that in intimidation. Intimidation is different than fighting...fighting is a fall back when the intimidation fails and a truly successful intimidater would never actually fight. Size and scariness becomes an end all here...not strength or agility, but size at all expense and it's in this category of creatures that you'll see the biggest variations in size between sexes in the same species. Polar bears fit in this category capable of using intimidation to scare other creatures such as wolves off a kill so they may claim it for themselves. Most male on male polar bear encounters end with one of the males backing off before fighting occurs...only extremely close matchups or extreme situations actually end in a fight.
There is one creature, now extinct, known as the giant short faced bear that might have seen a greater sexual dimorphism as far as size goes (my speculation). The giant short faced bear would have been a brutal opponent in battle, but it is most likely these creatures were scavengers and not killers, relying on their extremely strong sense of smell to locate a fresh kill and it's massively intimidating frame to scare off any competition for that kill (15 feet tall when reared up with overtly long arms and giant claws). Mate selection within the giant short faced bear was likely intimidation based, leaving the males most capable of intimidating the most capable of mating and a strong pressure on males to become larger and larger.