In Bitcoin, the chosen block is the one with the most hashpower in the linear chain behind it. Abandoned forks contribute nothing to that.
With GHOST, the chosen block is the one with the most hashpower descended from it, in all forks. This way all the forks contribute to a block's security.
With a linear chain, propagation time is an issue. If blocks are too short it's more likely that miners will find more than one candidate block, causing the work on abandoned blocks to be wasted. With GHOST, no work is wasted.
The original GHOST paper, which proposed the algorithm as a modification to Bitcoin, has some math to calculate, for a given propagation time, the block time that would produce equivalent security to a linear chain. It was significantly shorter.