BIP91 is only a mechanism to assure that all miners that accept Segwit2x will also signal for the "traditional" Segwit (BIP141). It has not directly something to do with the 1MB vs. 2MB debate. If BIP91 is locked in, all miners that run that client will have the rule that they only mine blocks on chains where all blocks are signalling for Segwit.

Indirectly it has something to do, as miners that support the agreement are meant to use the Segwit2x code, and in this implementation the 2MB hard fork will be integrated. But a miner can perfectly run a custom client with BIP91 and without the 2MB part (as far as I understand it).