I'm quite unskilled at poker and there are many plays I see in videos that I don't understand the logic for. Maybe there other like me who have questions like these.

Btn opens Q3s, BB calls
Flop: T43r
Btn says it's good to bet large here, bets 2/3rds.

What's the logic for this? I see a lot of small bet from button. I get the logic for the small bets
Btn has position and BB is going to have bunch of air and will have to over fold a lot of flops so small bet become very plus EV.

Btn also has a range advantage bc BB never has over pairs. I'm not sure how this plays into sizing tho. Bc yes, btn has OP but has lot of crap too, so idk if that's a reason for a big or smaller bet.