Road Riding – faster on hills than the flat

I have the completely opposite issue.. On flats I am pretty good, and on anything up to about 4% incline it’s comfortable. But anything beyond that I really struggle. I would think its cause I’m not exactly light, but my fatter mate who can’t keep up with me on the flats and slight inclines always drops me when it’s steep.

I think it’s because I just don’t ride enough steep hills, and it maybe uses slightly different muscles. I just find my power drops off really quickly when the road points upwards