Air is basically the same power, so having half the battery capacity, it runs half as long.
It's a bit like 'how big a heater to I need for my house'. The bigger the house (think bowl) and the worse the insulation the more heater you need. But the heater power itself is cycled on and off, so a more powerful heater is simply not on as long each cycle. I think people want to know how long a charge lasts, not the details?
The factor you've got in play with the 'heater comes on' mystery is hysteresis. If you set the thermostat on the wall to say 70 degrees and it's 65 in the room, the heater comes on. As the room heats, it shuts off and the room cools. Well and good. But if you're not careful it'll get to 70 spot on and get really confused. Trying to switch on and off very fast.....not good. Enter hysteresis. Here the heater doesn't click off until say 71 degrees and doesn't click on again until say 68, a 3 degree 'dead band'. It could be on or off at 69 depending. This stops 'fast cycling' which is a bad thing to have happen. Solo is like that. It won't turn on until it's a bit under the mark and won't shut off right away. In the thermostat case, this is clearly a temperature difference, computer versions, like Solo, often include a factor called 'reset' measured in seconds as well. Otherwise both compare the actual result (called 'process variable' in the trade) and compare it to the ideal (called 'setpoint') and establish a dead band around the PV. If you pull the cover off the thermostat in your living room you'll usually find a magnet that does this or a mercury filled switch that shifts 'over center' in older systems.
Take the Thermovape T1 and it's replacement, Cera LL. Cera kept about the same load size, but increased the insulation a LOT. As a result the heater was dropped from 30 Watts (a true battery killer with the smaller battery it had) to under 20 Watts with a higher capacity, bigger battery. This gave 4 or 5 times more time per charge taken together.
While I've never seen either of the S&B machines in person, I'm betting they're pretty sloppy with insulation and probably have a larger load? It's all part of the compromises.
OF