Why do lone pairs of electrons have a higher repulsive force? I was just wondering why lone pairs of electrons are more repulsive than bonded pairs of electrons.

I have read that lone pairs have orbitals which are shorter and more round, compared to bonded pairs. I assume, therefore, that this difference in repulsive force could be due to the increased surface area that these orbitals provide, which would cause more repulsive force to be directed towards electrons.

Of course this is just a presumption that I made, which is probably incorrect.
Bonding pairs are stabilized between two atoms. Since there's no atom on the other end of a lone pair, it spreads out more than would be if it were in a bond. Lone pairs therefore repel more because the charge density is spread out more, that is, it's bigger.


  I have read that lone pairs have orbitals which are shorter and more round


This is technically true in the sense that you'd want to have higher $s$ character in a lone pair orbital. But you're comparing possible ways to hybridize orbitals for lone pairs, as opposed to comparing lone pair atomic orbitals with bonding orbitals.