DD1is 22 months and she is wearing size 3, she is just about to move in size 4.

I bought some 'cheeky little soles' shoes for her and they are a medium which are 6-12 month or US size 4
and they are a bit big for her.

Apparently she has small feet and I never realised, they were so big when she was born.

ETA: If size 4 is 6-12 months then I would think that size 5 is 12-18 months which would mean that the size is pretty average for your DD at her age .