Cotton Rich Colourblock Sweatshirt (2-7 Years)

I bought this jumper for my son who is 27 months (2 years 3 months) in a size 2-3. Tried it on him and could not get it on over his head. So i exchanged it for a size 3-4 and this doesn't fit either. My son isn't an overly big 2 year old. Who are these measured on? I am now having to go back to store again to exchange it.

Bought online for my 20 month toddler - bought age 2-3 as he's growing out of his 18-24 month clothes. Because of the added grey pannel of the design I think this makes the fit a little awkward in comparison to the many other round neck M&S sweatshirts I've bought him. The torso is a bit narrow and it seems baggier across his shoulders. It is not easy to get his arms up and through the arm holes. Lovely bright coloured design and washes well.