This is a great question! "To stand erect" means to stand straight and tall; I'm not sure there's a comparable for that definition of the word. I don't think you can say one person stands more erect than another; you're either standing erect or you're not. The dictionary definition seems to support that:
- upright in position or posture:
to stand or sit erect.
So I might rewrite your sentence as:
The bunny on the right-hand side appears to be bigger because it is standing erect, while the one on the left is not.
There is little room for misinterpretation in your sentence, and "to stand erect" is a perfectly grammatical and correct phrase. It is worth noting, however, the second definition of erect:
raised or directed upward:
a dog with ears erect.
The word erect has a sexual connotation when used in this manner, so you may wish to keep this in mind (depending on your audience).