Why are ultrasound machines so big?
Beam resolution on an ultrasound is PURELY a function of the transducer, not the size of the box.   

Your assumption, though, is simply wrong.  There are small ultrasounds, plenty of them.  The way the machines have been traditionally used, though, did not demand small size, and a big, self-contained machined complete with display fit into the clinical model just fine.  Now that such things are becoming field-deployed, they are becoming much smaller.

The fact that much of the old technology is still around, and companies are slow to update to more modern designs is almost certainly due to the regulatory environment.  You can't just change a medical device.  The generic pulsed echo ultrasound system is a Class II FDA device, approved through the 510K mechanism.  You can see guidance documents for bringing such a system through the FDA process HERE

Changing an approved device means redealing with the FDA, sending in 510K modifications, subjecting yourself to re-review in some cases, etc.  There is a lot of "if it ain't broke, don't fix it" type thinking in the medical device regulatory arena.  

The device you chose is not in the FDA system, as it is a veterinary product (otherwise, I'd be telling you the date of the FDA approval to market), but they have their own regs.  For that example, you're probably just talking about the business model vs the cost of opening up a design cycle.  If they have no concerns about losing market share, the product will stay the same.  Though, the same company sells laptop sized systems.