Is my yoyo loach going to be tiny for the rest of its life?

I would suspect internal parasite or internal bacterial infection, but particularly internal parasites as they can show few symptoms in loaches other than a lack of growth and a bit of skinnyness. Treat with an anti-internal parasitic medication and see if this helps as this happened with both my clown loaches and my yoyo loaches in their first year ^_^v .