Disk platter sizes vary with drive speeds
Faster speeds equal small platters
Comment When is a 2.5-inch form factor drive not a 2.5-inch form factor drive?
A small form factor (SFF) drive comes in an industry standard 2.5-inch form factor case size. Inside that there can be one or two platters and which can be smaller or larger. Yes, they will be smaller platters than ones found in 3.5-inch form factor cases, but the largest of them can nearly equal a small platter found inside a high-speed 3.5-inch form factor disk drive.
The thing is that rotating a platter at high speeds is a hard thing to do because the higher the speed the stronger, and generally larger, the platter central mounting has to be, and the physically smaller the platter has to be so that the outer areas don't wobble and cause a data mis-read or worse, as in head crash worse.
What this speed:size relationship means is that a Seagate Momentus 7200.9 SFF laptop drive spins at 7900rpm and has a larger platter size than a Savvio SFF drive spinning at 15,000rpm.
This means that areal density comments about drives have to be made with reference both to the external case dimensions - 3.5 or 2.5-inch - and to the internal platter size. Generally such areal density statements don't refer to the platter size at all. A SFF Momentus 7900 might have 250MB/platter while the SFF Savvio 15K.2 has 148MB, but that doesn't mean the Savvio has lesser areal density. It has a smaller physical platter area and that's why its capacity is so much less.
In the enterprise storage area, a 3.5-inch Barracuda ES.2, spinning at 7,200rpm, can hold a maximum of 1TB on its two platters whilst a Cheetah 15K.8, spinning at 15,000rpm, can hold only 450GB on its two smaller platters. But then it is a much faster drive.
There are no generally known industry standards for platter size dimensions. We can't assume that Hitachi GST, WD, Toshiba and Seagate SFF 7900rpm drives all have the same platter dimensions, for example. Ditto at the 3.5-inch level. We can be reasonably certain, though, that at the largest capacity sizes in any form factor and low-to mid-speed rotation the largest possible platters are put inside the cases.
So remember, when talking areal density your mileage may well vary and you have to be making apple-for-apples comparisons, which you won't know unless you're aware of the platter sizes involved. ®