For example, if you fight 5 level 5 Bidoof in a horde battle, do you get the same amount of experience as defeating a level 25 Bidoof?
Applying this formula, assuming that your Pokemon is level 20, the experience yield of defeating 5 level 5 Bidoof would be approximately 67, while the experience yield of defeating one level 25 Bidoof under the same circumstances would be 312, if my math is correct. There may be some arbitrary level that your Pokemon could be at where it would gain the same amount of experience either way, but in general, no.