The shape isn't exactly the problem. Appearances were such meaningless and fleeting things, particularly when they are something that the both of them have long since had control over. Bodies were just vessels to house them while on Earth, hardly anything like their actual, honest appearances. Except now, Aziraphale is rather firmly tethered to his body in a way that he has not been before and that previous sense of control has largely slipped away.

"That's hardly the same! It isn't even comparable," he protests. It is a bit like comparing apples and oranges, he thinks. It's in the same general area, but completely different.

"I may be me still, but I do now possess a body that I don't always have full control over. At times when you were a snake, it was your choice and I don't imagine it had a profound effect on how you felt or, or, or reacted."

He makes a wide twirling gesture with his hand, clearly trying to articulate something he isn't actually saying in terms of how his body reacts to things now. He also understands that Crowley doesn't share his opinion here, judging by his haste to speak, but Aziraphale feels it to be important for him to understand the thought process behind the assumption.