Of course it is. Why do you think when you only fight one, it appears in a fixed corner [as to be able to fit exactly 4 in two rows of two] as opposed to the exact centre, like a normal "one at a time only" monster would appear [ie. something like Garland appearing in the middle even though his sprite is small as shown here]?
Size doesn't matter. You could fit four of any of the fiends into a combat screen but it's only possible to fight one at a time.

That picture means nothing. To whoever made it~ I can edit them too:

Fake

It's not possible to fight more than one at a time.