Smallest height 2.5" HDD is 100x70x5 (35,000 mm^3), max size 4T
MicroSD is 15x11x1 (165 mm^3), max size 200G
MicroSD is less than 0.5% the volume of 2.5" HDD, but 5% of the capacity. So MicroSD is an order of magnitude more dense than 2.5" HDD.
MicroSD still has to fit a little controller in there, so the comparison isn't particularly fair on flash. I expect heat management to be a problem scaling up though, apart from any manufacturing difficulties.
I was screwing around with writing firmware images to a micro SD card last night. Placed in a small USB adapter, the card got surprisingly hot after just writing a 4GB image to it. Fill a 2.5" form factor with the things and you'd probably start a fire.
MicroSD is 15x11x1 (165 mm^3), max size 200G
MicroSD is less than 0.5% the volume of 2.5" HDD, but 5% of the capacity. So MicroSD is an order of magnitude more dense than 2.5" HDD.
MicroSD still has to fit a little controller in there, so the comparison isn't particularly fair on flash. I expect heat management to be a problem scaling up though, apart from any manufacturing difficulties.