The trouble with a power deficit, as we saw in the first semifinal, is that the lighter-hitting player’s margin for error can become untenably small.
There are still ways to win — serve well, hit groundstrokes with great depth that can lead to short balls, defend like your life depends on it — but to actually gain traction in a match, the player has to do all those things at a high level, for a long time.
The more powerful player, all else being equal, can essentially reduce their strategy to “hit the ball hard and accurately.”
Barring a drastically excessive unforced error count, the bigger hitter does more damage with the average shot, and gets to play within their comfort zone while their opponent has to press into the red just to keep up.
So far, Rybakina has executed her power game at a high level, while Pegula has missed too many second serve returns and neutral groundstrokes in rallies, which is why Rybakina is a set up.