Miner A is able to process 100 M tx/block while miner B is only able
to process 10 M tx/block.
Will miner B be able to maintain itself competitive against miner B?
The answer is: it depends on the consensus maximum block size.
No, it depends on all of the variables that go into the mining profitability equation.
Does miner B have access to cheaper electricity than miner A?
Access to more advanced mining hardware, sooner?
Ability to use excess heat generated from mining productively?
Access to inexpensive labor to oversee their operations?
Access to inexpensive capital to finance investment in hardware?
The number of fee-paying transactions a miner can profitably include in their blocks will certainly eventually be part of that equation (it is insignificant today), and that's fantastic-- we WANT miners to include lots of transactions in their blocks.