Dann, you are neglecting the issue of balance, which is the key issue w/ large short-armed bipeds. Whether they bog down is not the issue, it is whether they _fall_ down. What can suddenly happen to your 'vertical equilibrium' in situations of uneven, steep, or low density footing has a much wider range of outcomes when you are bipedal.
It is an area of locomotion that the quadruped has a fundamental advantage in, making a wider range of substrate environments available to them; thus the possibility of refuge. Bipeds are particularly vulnerable to sudden, unpredictable changes in footing (vertical differential, traction, etc).
I feel it probable that the large bipeds not only stuck to firm ground, but probably firm ground that was well-compacted by large quadrupeds. And why wouldn't they?
I agree intuitively that sink rate in some 'densities' of substrate would be slower w/ the theropod foot, but the sink rate is not the metric that measures the likelihood of traversing a given area, except (possibly) under certain very unlikely and limited conditions. I actually have considerable personal experience in this area, having spent much of my life literally in swamps. This includes working w/ machinery; pickups and tractors; and animals (cows, horses, and dogs). Trust me on this; a cow can go places a man can't go, WITHOUT using his hands, and a mule bred from a Percheron horse can pull logs from a place an ox can't.
Which is why the old-time loggers used them, or so I am told. Even more to the point, I became permanently restricted to crutches some years ago, and have experience w/ both modes that very few humans can claim to have. This is because I am not very smart, and keep right on going out there... quadrupedia is even superior in machines; motorcycles can't go where the 4 wheelers can.
I can cite no studies...
... but can tell you this with confidence. A quadruped can climb steeper slopes, and traverse softer ground than a biped that cannot use it's 'hands'...
... therefore locomotive substrate can offer refuge from large bipeds, even if only part of the time. Daily and seasonal cycles would be factors, as would climate cycles. Relative body mass plays into this, I suppose, but not in any linear way. Also, some large theropods had longer arms than others. How long your arms have to be to be of assistance in critically unstable balance situations I don't know, of course. Intuitively, I would say arms much shorter than a humans is too short.
Dann Pigdon GIS / Archaeologist http://www.geocities.com/dannsdinosaurs Melbourne, Australia http://heretichides.soffiles.com ___________________________________________________________________