DayZ is a hardcore game thats the key difference. Its about realism and its part of what makes DayZ exciting and unique. Doing anything in the game takes a lot of time so when you are faced with a PvP situation its intensity is much higher than any other game.
The tire or jerry can sizes should not decrease. It is realistic, no way would you be able to fit a tire into any backpack. If the Devs really wanted to assist in this situation then maybe they can add a craftable tire carrying kit with rope or something. Hang 1 around your back (although this would be pretty uncomfortable in reality) or perhaps make a sled to be able to lug more than 1 tire around. But I doubt the latter would happen.
As it is now is not too bad. Getting a vehicle working is very much so an "end-game" thing. Its another task that you can decide to do and it shouldn't be easy.