According to Google, Cav is 5'9" and 154lbs.
Egan Bernal is also 5'9", but 130lbs.
Tadej Pogacar is 5'9", and 146lbs.
So in addition to training for that explosive sprint power, Cavendish is carrying 3.5 kilos more than Pogacar, and over 10 kilos more than Bernal. Given how much attention is paid to ~0.5 kilos on the bike, a 10 kilo deficit will end up adding up quite a bit over the course of a 10 mi/16 km climb. That said, I'd bet dollars to donuts that anybody in the Gruppetto can outclimb anybody on this forum.
(as a side note, I'm a little surprised that top riders don't have completely shaved heads/facial hair to save those precious last few grams going uphill)