Federer used to play with 90'sq and switch to 98sq, after that alter to 97'sq. Rafa alway played with 100sq, Djokovic switched from Wilson 95sq to Head 100sq back in 2009. Sampras and many players from his era played with very small frame, for todays standards, just 85sq. Sweat spot is way bigger and easier to play with bigger racquets.
His BH improved with bigger frame, but not significantly untili Ljubicic came in picture, who also had great one hander BH. So 2017. onward Federer is almost diffrent player, thanks to his improved BH, to bad he cannot be any younger. His movement and speed suffer over the years, hence less and less IO FH utilization in his game. For sake of longevity racquet has been changed to try improving BH side, so he wouldn't have to run around his BH and play constantly IO FH, which was his strength in younger days. It takes time for pro, especially in his age, to dramaticly improve and change one of the groundstrokes along with diffrent/bigger frame.
This year grass is realy slow, but thank god bounce is low which is essence for grass. In 2006-2008 period, Nadal prepare for grass on damn clay !! They make him competitive by making conditions who favor his game, slow and bigger bounce than should be allowed on grass. When bounce once again become very low, he couldnt even see second week of Wimbledon after 2011. And he was still in his prime. And they never changed fu%@g clay