Well, it's not like Spyro was informed of how long their shapes would hold at minimum, but he can see how it would be pretty catastrophic if the elder dragon suddenly returned into his true size without warning or a way to delay it.

"Okay, I really don't know how soon this might wear off so... I'll go in to get some food for us and be right back."

Plus that leaves plenty of time for anyone else to bother Ire for questions, a dragon transformation, or even throw snowballs. And Spyro trods off to do exactly that.