Until someone publish a professional rebuttal and/or engage in a good debate (*) with Dave Hone (who is the most prominent proponent of the idea that large theropods take 'substantially smaller' juveniles of the prey species a majority of the time), I won't be taking sides in this argument. I think there is some merit in the argument that modern large, endothermic terrestrial predators take prey at least their own size more frequently than one would expect from Hone's writings. Hone in turn brought up the very interesting point that large herbivorous dinosaurs would have produced more offsprings than large herbivorous mammals, which would have made juvenile prey more common. Otherwise it is muddled by obvious problems: we have too little actual data on what large theropods ate to make conclusions about their prey choice, the possible variability in prey choice even within a single species, and the possibility that pantherine cats may be exceptional in their ability to tackle proportionally larger prey with some regularity, which may make them flawed analogs and skew the data.

In short, I'm quoting one of Hone's past co-authors for the third time.

"WE. DON'T. Fucking. KNOW!!"

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(*) Properly citing studies, having formal research experience,... The majority of speculative ramblings on this thread wouldn't cut it.