It takes two pounds of extra cooling to keep the M1 Ultra Mac Studio from frying
Did you wonder why Apple's Mac Studio weighs a full two pounds more (7.9lbs versus 5.9lbs) when you choose the M1 Ultra model instead of the baseline M1 Max version? There's a simple explanation for it — and no, it's not that the extra silicon weighs more. Apple explained to The Verge that the M1 Ultra variant uses a larger and heavier copper cooling system where the M1 Max can make do with an aluminum heatsink. The 370W power supply and other components weigh virtually the same.
The cooling is crucial to the Mac Studio's small form factor. Much of the internal space is devoted to the thermal module and the vents it needs to expel heat — the choice of metal could make a big difference in the ability to transfer that heat away from sensitive electronics. Copper isn't necessarily better than aluminum (the design also plays a large role), but it's frequently used in more aggressive PC cooling setups and could help Apple spin fans less often.
You probably won't worry about this much unless you're routinely carting your Mac Studio around. However, the weight difference also helps explain why you won't see a MacBook Pro with an M1 Ultra any time soon. As the Ultra is effectively two M1 Max chips linked together, it generates considerably more heat in addition to its greater power demands. Even if battery life was unaffected, Apple would likely need a bulkier (and possibly noisier) cooling module to keep the laptop's temperatures in check.