Re: Hmmmm I'm guessing the 450vDC in is from PV panels
As usual - this is the reason why good engineers get paid quite handsomely.
The problem is in the details - size and efficiency. In order to get size down, you'll need to get switching frequency up (smaller capacitors, smaller inductors). But that goes exactly against getting efficiency up (every state change induces losses).
While you can apparently without too much trouble switch 5 V at 1 Amp at around 1...2 MHz (and, at this level, the difference between 85% and 95% efficiency is negligible), doing the same with 450 Volts and 5 Amps (or 240 V and 10 Amps) tends to be a bit more difficult.
I got a couple of 5 kW boxes at home - picnic cooler is a bit exaggerated, though. Squeezing half that power into the size of a laptop - well, it'll depend on the laptop, but it looks feasible.