If memory serves, the amount of roaming data you get is computed on the basis of 'what would you get if you paid roughly the same amount of money to a typical provider in the country where I am travelling', with some caps in both directions.

I live in Italy which has low prices, so I get 50 GB/mo for 8€ (and 200 GB/mo during the summer). Last summer I travelled to Austria and Czechia, I had 4GB available in both countries, and indeed checking their biggest ISPs' landing pages it looks like 8€ wouldn't have bought me even a gig! So 4GB is probably a mandatory minimum.