I would say 17-18" for 1 y.o. is about right. I base my models for 2 y.o. on 21" chest measurements. I noticed that children sweaters tend to be too wide, almost square, and too short to cover the upper body of an active toddler. I mean, it's a sweater so the idea of warmth shouldn't be foreign to it. Also, their puffy sleeves do not fit regular jacket sleeves. With those sweaters, even if a child grows and will be able to wear it longer, the sleeves most likely will be short before sweater is small.
So, I make longish, 'rectangular' sweaters with about 2" ease and sort of narrow and longish sleeves that allow for some growth (for example, make the cuff that can be folded).