"Well it's like this, you see we built this station, cost us 100t credits, a million people live in it, we designed it for the ships of the time, we didn't imagine ships getting so large, but now we can't kick out a million people, where are we going to put them for a start, and redesigning the letterbox to take larger ships means completely dismantling the station, have you any idea how much that is going to cost and how long it's going to take. And that's without doing the same to every other station in the system!"
You see I don't think we should just try and retrofit all the old stations, you would never do it in real life, and yes I know it's just a game, but I like my universe to be consistent, even if it is just a game. The Panama Canal is an example of this in real life, the old canal locks are to small for the new ships, some of those things are HUGE, but they don't just rip out the old dock, that would disrupt traffic for decades and cause no end of problems, so they built a series of parallel locks and new canal.
So new stations could maybe have the new larger slots, old stations stick with what they have and pilots of big ships either take a bit more care docking, or fly smaller ships into the stations with smaller slots if they can't handle it the way it is.
Oh yes, the mail slot isn't really small, your flying a frigate sized ship, everything looks small!
Sorry but thats just a bad way of thinking of it. First I'm not saying to kicking people off the station for a retro fit, second its bad design floor to only have one way in and one way out, if the thargoids or other extreme terrorist factions attack a station and concentrate there damage on mail slot then the people will be trap with no way out, with a retro fit CG it will fix this problem and a good excuse by adding extra slot. just like air fields and air ports has more than one air strip for takeoff and landings for safty.