They seem to go from size 1.5-2 to a 3yr with nothing in between. For a tall, but short legged, plump toddler what should I be buying? How big are the size 3s? She has one pair of 12-18mths which she can still wear, and a 1.5-2 which fits ok but not sure how much longer it will last her so if I'm buying winter stuff and want it to last is it worth going up a size or is the difference too huge?