Some guy in Reddit did some testing with a level 1 LB, and discovered that it is apparently the combined weapon damage that is the biggest factor for a Limit Break.

He tested it by having a full DL and +1 relic party, and they dealt 5.5k damage on a melee DD LB with everyone wearing their gear.
They went naked bar for their weapons and got the same result.
When only one was using his relic and the rest with level 1 weapons, the damage dropped to 1.8k. This was tested again naked, same result.

And it doesn't matter which DD it is coming from, a DRG with a 55 GC weapon will LB as hard as a relic +1 DRG who is in the same party. It literally is 'The power of the party' kinda thing, not the individual it seems.