Right now, many miners purposely leave blocks empty, because the advantage of a quick, empty block outweighs the fees from full blocks.
The more “full” blocks are, the more likely miners will prefer to mine full blocks, rather than quick blocks. Bitcoin unlimited will result in large miners upvoting to the max… at no cost to them, followed by a stochastic mining pattern with lots of empty, and lots of very large blocks.
This pattern will not increase capacity, but it will increase fees, the advantage to large miners — while reducing overall network health.
Segwit, which is a gradual increase to maximum block size (and yes, it is an actual increase — please read the code), is much more conservative and healthy for the network.