The problem with general statements like “bet bigger/smaller on twotone boards” is that they don’t consider what our betting range is built around.
Eg on low boards, the aggressor’s betting range is usually built around overpairs. So we do in fact see smaller bets on twotone boards, simply because these overpairs are worth less when the board is drawy. We would rather size down and get called by more low equity hands, instead of sizing up and getting called by a range that has so much equity against us.
That is, a small bet vs a weak pocket pair generates more EV than a big bet does against flush draws.
On higher boards however, we often bet bigger on twotone boards instead. This is because the types of hands we are betting are very different: we have more marginal hands like underpairs and middle pairs when the board is high, and these hands are more interested in betting when the board is rainbow - once again because they have better equity when called. So we carve out a small bet range around them on boards like KT5 rainbow.
On a board like KT5 twotone however, we are typically not that interested in betting hands like Tx. So there is no reason to use the small bet, and our betting range is instead built around hands like TP+ - hands that mostly want to bet big.
Try giving this a watch:
Why Good Players Don't Bet Big on Wet Boards
https://youtu.be/a1O75yVsa4M