to some exten neather and both.
lighter rounds do kill well and so do larger rounds.
you have to factor in bullet disine,weight,speed,section density, and penertration.
if light fast bullets were the best killers we would all hunt deer with some thing like a 220S.

ex a 220s 55gr moving 3170fps @100yd E @ 100yd is 1364 comped to a 45-70 405gr moving at 1168 fps and a E of 1227.
now the 220 is faster and has more Flbs of energy. is it a better killer of big game than the 45-70. i dont think so.
i would take the slow heavy over light and fast any day. from the game i have shot the heavy does better.

and yes i would take the 30-30 over a 243 any day of the week.
L. O. D. Charter member and L.O.S.