Lower weapon heat, higher range.
Light (2 and 5 rating) Clan LB-X have longer range profile than Clan UAC, and heavy (10 and 20 rating) Clan LB-X generate significantly less heat than Clan UAC per shot. This is true even when using the shell ammo instead of the cluster rounds.
Inner Sphere LB-X autocannon are
all longer range than their standard and Ultra companions, and the 10-X and 20-X both run less hot than the UAC equivalents (per shot, again) by an even greater margin than the Clan LB-X autocannon. Again, this is true regardless of cluster or shell rounds. Additionally, the Inner Sphere LB-X autocannon, while heavier than the Inner Sphere UACs (LB-10X aside) are all smaller in terms of critical hit slots.
Also, you really shouldn't be comparing half of the LB-X function to all of the UAC function. The option to choose between standard and cluster rounds with the LB-X is roughly equivalent to the option to choose between firing normally and double-tapping on the UACs.
The LB-X ability to swap between cluster and shell munitions obliterates the main complaint most players that dislike LB-X autocannon have against the weapon type to begin with- that they're relatively weak when the target still has armor. Being able to swap ammo means firing shells when the target has armor you need to break through with more punch to one spot (like a UAC being fired without using the jam-inducing extra shots) and then switching over to cluster rounds as soon as the armor is gone, so that you can detonate ammo, break weapons, and do improved overall damage (which the UAC doesn't have the option to do).
As for 'bigger', I don't know why the Inner Sphere LB 10-X is the only LB-X autocannon that is both smaller and lighter than its standard autocannon equivalent. Best guess says that way back when, FASA realized after introducing the IS LB10-X that making LB-X cannon all lighter and smaller would obviate the standard autocannon entirely and then failed to retroactively make the LB-10X bigger/heavier (or perhaps opted not to correct that for the sake of not having to redesign everything that already had an LB-10X).
Edited by Quickdraw Crobat, 01 August 2014 - 12:57 PM.