Imagine a situation kind of like Zootopia with many different creatures all living in one environment, some significantly larger than others. These species aren't necessarily furries like in Zootopia, but you still have size difference such as an elephant vs. a mouse where one species is a hundred times the weight of the other.
All the the species are equally as intelligent and competent, save for physical differences, and the environment has been built with some attempt to accommodate all species.
These species have been living together for over a century, and in that time their technology has developed to a level generally similar to, or ever so slightly greater than, our own. For now, assume that lifespans are somewhat similar (the 'Elephant' likely will live a good 10 to 20% longer then a mouse, but our 'Mouse' species still get a good long life, unlike actual mice).
It would be great if all these species lived together in harmony, but that's not how life usually works out. Humans have always taken any advantage they have had, in numbers, size, or superior technology, to put themselves in a position of power over other groups, we have seen that in interactions between races, nations, and sexes. We have gotten better at it lately, but the tendency is still there.
If these creatures in our theoretical Zootopia like structure are still human like, they will still likely compete for positions of power, and with species being such a significant divider they will likely rally around similar species and ultimately some species will try to take power over the other. Now they may not completely dominate and abuse the other species, but likely one species, or group of species, is going to tend, on average, to be in positions of more power or wealth or otherwise have some degree of 'privilege relative to the others. I'm trying to figure out who gets to be top dog (...pun not originally intended, but approved of).
At first glance it looks like your 'Elephants' would clearly be the ones to dominate, their vastly superior size and strength would allow them to physically dominate over smaller species and force their will on the smaller creatures. If this were during the dark ages, with feudal nobles and peasants, they likely would rule by virtue of physical strength.
However, in the modern era the 'mouse' starts to have some advantages. With more and more of our jobs being intellectual, or service, positions that require limited physical strength the mouse becomes able to work and earn profits as well as the elephant in many situations. A mouse can answer phones in a call center or program just as well as the 'elephant' can. However, being so much smaller they have far less needs, they eat less, can live in much smaller homes, and generally can live off of a fraction of the expense something as large as an elephant would need.
This could mean the 'mouse' can work for a fraction the pay and put the 'Elephant' out of a job, or maybe the 'mouse' works the same amount and simply has a far larger disposable income after covering their basic living expenses. Either way this puts the 'mouse' in a much stronger economic position.
Since they will live in some form of a democracy now the much larger number of mice to elephants alive would give them a stronger voting block as well. Then again the 'elephants' were likely in a stronger political position a century ago when all these species came together to form a democracy and shared living spaces, since their technology wasn't as advanced and the physical strength of larger species put them in a stronger negotiating position. Perhaps the elephants insisted that their constitution adjusted voting power by species in some manner to compensate for smaller species outnumbering larger ones to avoid small species having such a dominate voting block?
So ultimately which species ends up on 'top' in a modern multi-species living environment, the very large, or the very small?