Too much cargo space breaks the game" keeps getting claimed in comments under Panther videos, but when prodded at it, nobody can actually make an argument about how that would break the game
"Break the game" is a bit much. There's not enough finely-tuned game balance in Elite Dangerous to worry about that.
But there are a bunch of implications to having anything much larger than the current Panther Clipper design which Frontier might not have wanted to bother with.
1) The larger the cargo space a ship has, the greater the gap between its laden and unladen range. So it becomes harder to stop it either being painfully slow with cargo, or faster than the Mandalay without. Piling on additional hull mass helps (and they have done with the PC) but putting too much of
that on then has implications for its thruster performance, what happens when it rams another ship, etc.
2) The size 8E thrusters have a mass limit of around 3300T (and same with 8D engineered with dirty drives). So a ship with a total mass higher than that in "normal" configurations (you can exceed it on the Cutter, but only by doing some very bizarre stuff) probably needs size 9 thrusters, and Frontier might not have wanted to introduce an entire new size class of module (with possible knock-on effects elsewhere) for one ship.
3) Game balance is broken, sure, but a 5000t Panther Clipper would allow use of the "bulk flood" undermining method at a rate of around 6000 control points per hour. That's a specific type of breach of game balance - it makes undermining of systems in Powerplay feasible - which would be very unpopular even with people who don't care about hauling at all.
4) The Fleet Carrier doesn't have the same problem (the Fleet Carrier stomped all over a whole bunch of balance issues on its introduction, but not this one, and they weren't generally ones anyone cared about) because the balance issues with large cargo capacities aren't about storage, but about throughput: you still have to load and unload the Fleet Carrier, which means twice as many supercruise trips (though shorter ones) and twice as many dock/undock cycles, which means that it's only marginally faster than flying the cargo in whatever you're using to load/unload the carrier in many cases.
(The FC obviously wins out when it comes to larger distances - nothing can come anywhere near close to competing with it once you get to the 1000 LY+ range, which
does mean that Frontier can't easily introduce any long-range hauling tasks)
5) All balance is relative. If there's a ship which can haul 5000t (so maybe 25000t/hour on short hop routes or carrier loading/unloading), guess what the
next cargo-hauling-based activity is going to be balanced around CMDRs being able to do in terms of its tonnage requirements. If we'd had a 5000t Panther Clipper released a year ago instead of the Type-8, all colonisation tonnages would be about 5x their current value on the assumption that everyone was going to be using the Panther Clipper to do them. So a much bigger PC makes it a lot quicker to do all the existing hauling (wing trade mission in a single trip? don't mind if I do!) ... but just means that the
next hauling task to be introduced will be balanced around its existence, and anyone flying anything else whatsoever can't do a thing about it. Increasing cargo capacities is just a vicious cycle with increasing cargo hauling
requirements which we can't as players actually win.