depends on how you define relevant.
if you look at it from a gameplay perspective, you could also settle down and call most parts liberty city wasted space. you could condense the locations of the gameplay to an island about the size of brooker, with a little space in between to give the illusion of actually moving between them.
to deliver a certain feeling to the player, both gta 4 and san andreas did their job very well. liberty really gives you the feeling of an nyc-ish metropolis with vast urban areas, while san andreas delivered the feeling of travelling through a state, especially thanks to the "wasted space" - I'd rather call it landscape, as this adds so much to the illusion of a whole state.
of course, again, condense it to the gameplay-relevant areas and you can cope with a fraction of the area used.