User blog comment:NASCARfan0001/Phineas and Ferb:Summer Belongs To You/@comment-1896159-20100804014839

There's still the possibility that SBTY came AFTER Christmas Vacation. Yes, I know Phineas said that their vacation was "more than halfway over", but if that was the case, the episode would be taking place in mid to late July instead of the Summer Solstice, which is generally in June.

@Phinbella ferbessa: Perhaps Isabella's the one that got shorter in Christmas Vacation and taller in SBTY.