why are toddler vests thinner than baby vests?

W is toilet trained now so i can't get her baby type vests with poppers. 

the vests that are big girls ones all seem to be really really thin material. Like thinner than her knickers.

does anyone know anywhere where you can buy ones that are a bit thicker? More like the thickness of the ones you get that fasten between the legs? 

ive tried Asda tesco and matalan and they are all the same.