why does a 3/4 lb rope feel harder to use than a 3 lb rope?

I'm not sure if it's just the speed is too much and that's why;but context:

i try to skip every other day in the mornings, mostly with a normal speed rope, but I try to alternate sets with a heavy rope. I was doing ok with the 3 lb rope, the rotation is slower, but i definitely feel the burn. Doable though, this was my usual routine. I wanted to change things up by having sets keep switching weights (go from speed rope, to 3/4 lbs, to 1.5 lbs, to 3 lbs and back again)

but with the 3/4 is it the rotations? cause the burn on the shoulders is insane and immediate...

I'm using the crossrope stuff if that helps