They can make up the size difference by having better frequencies. They are planned to run on 90 second intervals instead of the planned minimum frequency of TRs of 100-110 second intervals. This makes up for the smaller train size.I am not concerned at all about OL being full on day one; that won't happen. The concern is that the line isn't future-proof enough.
Regarding the capacity, I can happily live with it only losing 10% compared to the traditional TTC design. However, if the OL trains are 15% shorter and 15% narrower than TR trains, then they can't have 90% of the TR capacity unless some kind of spatial magic is involved. (1 - 0.15) x (1 - 0.15) = 0.85 x 0.85 = 0.72, or 28% capacity loss.
And I get it that the lower capacity of one train can be compensated by running the trains more frequently. But the TRs can run more frequently as well, after the signal system overhaul and maybe adding some exists and widening some of the existing exits. That's a nontrivial investment, but a lot cheaper than building the next line.
For TRs, or any trains of similar size (no need to get fixated on the TTC's current equipment), frequency improvements with capacity growth is a future opportunity. On the contrary, the stated design maximum for OL will remain the maximum forever, because the greatest physically possible frequency is already counted in.
I hardly can imagine a situation where the ultimate capacity isn't nearly proportional to the train's floor area.