Hello DIBYes the size difference between the DC08/DC08TW/DC19/DC20 and DC21 to the DC12 is amazing. I wonder how much smaller the DC05 to the DC12 is!? Wish the DC12 would come to the UK sometime!
Although I like Dyson Canisters I'm not one for using them to clean carpet, I for one think Uprights with a brush bar does a far better job. I normal canister head does not agitate the carpet and pick the pile up unlike uprights do! If I had hard floors everywhere then I would use a Dyson Canister, although I do own a DC05 for car cleaning! Not everyone wants to be getting the hose out on an upright every time to clean under low furniture especially if you’re doing a quick clean around!! Take the DC03 slimmer than the DC18 but not as powerful yet that would lie flat to go under beds or low furniture for deep down brush cleaning, quick and easy. The DC18 is getting there the head can get into places other models can not but still too bulky. Both Canister and Uprights are becoming similar to each other these days, yes canisters get under low furniture better and with the DC21 Motorhead offering brush bar cleaning!
The turbo nozzle on the DC12 is air driven, although Dyson has improved so much in the air driven nozzle for it's canister I still don't think they are effective as motor driven ones! It does look like Dyson is going in the direction of using a 2 motor system for its uprights just looking at the DC15, DC17 and DC18 Slim which all use this principle!
Answer to your question how any upright could fit under a bed, you just have to look at the DC03 (UK) and other makes like Sebo X4 and Felix, the Felix like the DC15 and DC18 uses a separate motor for it's head making that flat so it can fit under low furniture, the X4 is very slim profile like the DC03 so laid flat can go right under low furniture. Vacuums have become heavy and bulky and Dyson has the technology and know how to change this, you just have to look at the DDM and the DC12. Slim bins have been used before just look at the DC03 and DC11 (Twin Root Cyclone bin).
Don't get me wrong I'm a very big Dyson follower but James Dyson needs to address these things, to push his technology forward solving more problems. People want effective, powerful easy to use but light versatile vacuums. The DC18 is one of them but there is more to come! Dyson launched this model to address such things as weight and bulky uprights so he knows there is an issue. Again the DDM is another issue he is addressing.
DC18