Not to detract from the other very fine answers, but I suspect that there are variables you could modify. For instance, if the composition of the core were different to the Earth's then you might have a sufficient magnetic field and increased density. This might allow a smaller planet to retain atmosphere and have a similar surface gravity to Earth.
For the sake of argument, let's say that the planet is half the size of the Earth but the other factors are broadly similar - the atmosphere, gravity, temperatures, length of day, etc.
The question then is whether the reduced space alone would result in evolutionary selection pressure towards smaller creatures.
Personally I think that it's unlikely. Smaller populations, maybe. But I don't see any particular impediment to 'normal' (Earth-normal that is) sized creatures being the norm. The balance we have reached seems more to do with the direct factors: gravity, atmosphere, temperature, etc. Indirect factors like room to expand and so on would seem to be more likely to result in smaller populations or reduced diversity than to produce smaller individuals.
If you were to find a rock like this, Terraform it, populate it with an Earth-native ecosystem and let it develop independently for a few hundred thousand years and things probably wouldn't change all that much more than on a world more like Earth in size. Maybe a little less, given the reduced opportunity for geographic isolation.