You make an excellent point and I'm sympathetic to MPs who have to represent that many more constituents. One way it can be addressed, I think, without adding more MPs is, perhaps, providing for greater administrative support, more money so you could hire more people. That would help you discharge the responsibilities of MPs who have more constituents. That's how I think it could be done.
There are other jurisdictions that have ceilings, the United States, for example, and they have greater support for their senators and congressmen.