Manpower is not as important as training, which is not as important as technology and strategy.
Russia is already on its “4th army”: best trained forces are gone, reservists are gone, prisoners and noob recruits are gone… they’re now fighting with skeleton crews and mercenaries.
Ukraine on the other hand, has spent the time building up expertise in state of the art drone warfare strategies… which are completely novel and haven’t had a chance to get history books written about how to beat them.
It isn’t as simple as “manpower”.
The one thing that Russia has, and hasn’t used yet, is nukes… but that would open a full box of nasties.