Learn Finnish: Miksi et valinnut isoa painoa? - Why didn't you choose a big weight?

valita (to choose) is a Type 5 verb. To form the active past participle for this verb type, the t before the final -a blends (assimilates) with the -nut ending. The t becomes an n, giving us valinnut instead of valitnut.

Why is the verb valinnut used here instead of the regular past tense valitsit?

This is a negative past tense sentence. In Finnish, you form this by using the negative verb matching the person (here, et for 'you didn't') followed by the active past participle of the main verb (valinnut). The main verb doesn't take the normal past tense 'i' marker in negative sentences.

Why do we say isoa painoa instead of iso paino?

The object of a negative sentence in Finnish almost always takes the partitive case. Because the action of choosing didn't happen, the object isn't completely affected. Therefore, the basic form iso paino changes to the partitive isoa painoa.

I know paino means weight. Does it refer to a gym weight or body weight?

It can refer to both! In this context, it likely refers to a physical weight at the gym. It comes from the verb painaa, which means to weigh or to press.