No they can't. Space will always be used. Hideo Kojima said that he though he'd have it easy space wise doing MGS4 for the PS3 but then the high definition textures take up a lot more space than before and what really eats space is the better quality sounds. To create cars and bikes in a single game that is also a simulation would require a physics engine or magnificent proportions to pull off. Yoiu can't have a race with two seperate physics engine operating at the same time, one for cars and one for bikes. You'd need a single engine that would simulate cars and bikes together. Secondly cars and bikes don't race together, again with GT being a game in the simulation gener that would greatly take away from the realism. Something would have to give, you couldn't have cars bashing riders off bikes, a lot of car manufacturers would pull out for that. So how do you do the crashes, do you make it so that bikes simply cannot be knocked over, yeah, that's good. Secondly, bikes are generally a hell of a lot faster on your average race track than cars. I think it was 5th gear where they demonstrated that a 10k road bike could only be beaten by a 300k+ supercar or something like that. So what your left with is a game with bikes and cars but both are seperated, so they simply do not race against each other, at least not directly. Wich then leaves PD one franchise short of what they had.
Personally, even if you took away thoes problems, PD should imo keep the two franchises seperate anyway. Cars and bikes simply do not race against each other, and I woul not like to be racing in the GT All Stars in my McLaren F1 with a grid half made up of superbikes, or even one. Talk about killing the sense of immersion realism wise.