Samarth 619

#1 If we analyze the facts closely, we'll find a good reason. Before we had 67x42.5 combination, all other 599cc combinations used before were smaller bore & longer stroke (like 65.5x44.5).
Additionally, all these 4 bikes have been keeping the same Bore x Stroke since 2004 onwards, except for YZF R6 which adopted it in 2006.

And amazingly, this particular R6 claimed to be the highest revving bike with redline around 17.5 krpm. (Although later it was found that the tacho used to read slightly optimistically).

The point is that any more short stroking and big boring will result in higher redlines. And in such a situation, the entire machine has to cope up with the additional revving... The valves, the cams, the crankshaft, everything! And today's tuning potential is enough to extract maximum from an engine, even without modifying its Bore-Stroke.


#2: GSXR 750 dates back to the 70's and 80's so probably they thought they should retain it. And one thing: odd man out capacity bikes have made very good sales like 675, 750, 848, etc.
There are multiple known and unknown reasons for it. I'm talking about the road. GSXR 750, in the past has sold better than its smaller 600 and bigger 1000 twin.

Why the competition ignores this segment could be due to lack of proper racing in this segment, who knows?

And about your query about Litre class rules, well, IIRC, it was a maximum allowed 81mm bore. Also, under 1000cc inline 4/ V4's and under 1200cc V2/ L2 are considered race-able. But this is old so it might have changed.