If you're mad that the etymology of "microtransaction" doesn't match its current usage, you're going to be apoplectic when you learn about 90% of English words.

Probably called that because it's smaller than the transaction of buying the game

You would think so, but that’s not how the term has been used for over a decade. It has always been a marketing tool. Any in game transaction is called micro transactions. For what it’s worth most games with so called micro transactions are free. Look at https://www.pathofexile.com/shop/category/armour-effects, 100 coins is $10. there is nothing „micro“ about those prices