It's not as simple as 3.5" drives being heavier.
The long-abandoned but then-excellent PC Guide has a page saying:
"Reducing the hard disk platter's diameter by a factor of two approximately quadruples its rigidity."
Of course this is ignoring differences in the material and thickness. Laptop drives use glass platters (which are more rigid, all other things being equal) but they are thinner than in desktop drives.
According to that rule, all other things being equal, a 2.5" platter should be 2.24 times more rigid than a 95mm platter. So even if the 2.5" platter is half the thickness, and made using the same substrate, it still has the edge.
I'm not denying that the casings of laptop drives are much thinner, quite possibly by more than enough to outweigh that rigidity rule (as well as being the same material), but 2.5" server drives should have thicker casings than laptop drives.
The long-abandoned but then-excellent PC Guide has a page saying:
"Reducing the hard disk platter's diameter by a factor of two approximately quadruples its rigidity."
Of course this is ignoring differences in the material and thickness. Laptop drives use glass platters (which are more rigid, all other things being equal) but they are thinner than in desktop drives.
According to that rule, all other things being equal, a 2.5" platter should be 2.24 times more rigid than a 95mm platter. So even if the 2.5" platter is half the thickness, and made using the same substrate, it still has the edge.
I'm not denying that the casings of laptop drives are much thinner, quite possibly by more than enough to outweigh that rigidity rule (as well as being the same material), but 2.5" server drives should have thicker casings than laptop drives.