I bought 4x (1TB->4TB the storage for half the price after my SSD died after 5 years (thanks samsung), what you mean they 'stopped being bigger'?
Sure, there is some limitation in format, can only shove so many chips on M.2, but you can get U.2 ones that are bigger than biggest HDD (tho price is pretty eye-watering)
By stopped getting bigger I mean people still think 4TB is big in 2025. Just like 2010 when 3/4TB was the max size for consumer storage devices. u.2/u.3 is not consumer yet, unfortunately. I have to use m.2 nvme to u.2 adapters which are not great. And as you say, low number of consumer cpu+mobo pcie lanes has been restricting from the number of disks side until just recently. At least in 2025 we can have more than 2 nvme storage disks again without disabling a pcie slot.