Dont know it this is really plays a huge difference, but I've read that in Joshua vs Ruiz first fight Joshua weighed unusually a lot, compared to his regular weight. Joshua even said that with extra weight he wanted to be stronger, plus he mentioned that he chased Ruiz instead of cutting corners. As if he was clumsy and used wrong tactics.
During their second fight Joshua was much leaner. If I found correct numbers, he was 12lbs lighter. Is this weight difference significant for a heavyweight ?
It wouldn't be possible to accurately calculate whether x amount of weight makes a difference, because it would completely depend on the circumstances, and the fighter themselves. Some people will perform better without additional weight, and others will prefer to be carrying a bit of timber. Joshua, it probably suits to be somewhat heavy, because he's a big puncher, and not particularly that great at the technical sides of boxing, which usually require a little more finesse.
However, if you want to compare weights just look at Usyk coming into the fight with Joshua, we was 15-20 pounds lighter than Joshua if my memory serves me correctly. I know there definitely was a big difference, since it was a major talking point before the fight, that Usyk would struggle with the power of Joshua because he's used to fighting at a lighter weight, and is significantly lighter than Joshua. Obviously, we know that if it did make a difference, it didn't change the outcome for Usyk.