The answer to that is holoforms. Basically, they can create tangible holograms that they can use to interact with humans. With that, they could stay in their vehicular form attempting to keep disguises, as they have been doing. This also allows them to conserve energy as they don't have to move around as much. So basically they interact with holograms that look like them as humans (I've searched it up and can give you pics if you want) and only fight when absolutely needed (if someone's life is on the line). Now the Decepticon will be the only problem, seeing as they're less big on the whole disguise thing.

Also, they could have depleted Energon levels, which would increase their need to search the area and find more, as well as cause them to think twice about fighting.

----

Or I could recall the characters and use a more human-sized transformer. Someone like Bumblebee or Frenzy/Rumble, or even Wheelie.