FLop: Smaller sizing likely widened V2 overcalling range (FD, SD, pp horribly mining one more street) as it still kept your range slightly more vulnerable to it (you weren't high beaming AA KK AK) and was cheap enough to just call it against 2 players OOP - while this didn't induce a raise, I think larger sizing would get a fold from one of the two Vs (or both) which we don't want so early in the hand, so I thought 55 was cool.
Turn: Though less likely to get calls from both on paired board, I'd say a similar relative size (145-155) keeps both Vs in a similar spot V1 might hold Axss and easily call now with reasonable SD chops/wins and if not that strong, the 3 really didn't change much else for any non draw he holds. Bottom line is, it doesn't matter anymore, we just cannot pass the threshold for a V1 call which at the smaller sizing COULD suck V2 to the river (if he's awful)
River: Perfect card, but can't get value by checking and PSB will look wildly strong and will get too many folds. A little larger sizing here since flushes won't be raising anyway and Ax may be calling just the same.