Why is grab so expensive?

My bf and I are living here 3 months and order our dinner from grab quite a lot when we're working.

Yesterday we had some greek salad with bread and sauce and he had dumplings and it cost 650 baht

We don't mind paying this for non local food but the portions were TINY .. By far way more expensive than anything in Europe.

How do you avoid getting ripped off on Grab and why are the prices so vastly different? It wasn't even a fancy place, just normal...