Credits are also much easier to earn in larger quantities proportionally once you've obtained larger vessels.
But not in any way proportional to the rebuy cost of those vessels. The bigger your ship is, the longer it takes to earn its cost back, often by a long way.
Trade: Sidewinder can carry 12t in a shieldless configuration. At 4k profit/tonne, it will take one trip to obtain multiple rebuys.
Cutter can carry 792t in a shieldless configuration. At 4k profit/tonne, it will take multiple trips to obtain just one rebuy.
Combat: an Engineered Sidewinder can hang around in a High RES helping out the cops at no real risk to itself whatsoever. An hour of that will get tens of rebuys.
An Engineered Cutter can of course snooze around in a Haz RES near-instantly splatting any NPC pirates it sees. After an hour of this, it might be a tenth of the way to earning a rebuy.
(Of course, an engineered FAS could likewise indefinitely dismantle NPC pirates in a Haz RES ... for a fraction of the price and rebuy)
Exploration: A Sidewinder and a Cutter earn exploration data at the exact same rate - actually, the Sidewinder with its better supercruise turn rate is probably slightly faster. A single terraformable world HMC will get a whole bunch of Sidewinder rebuys ... you'd need to get about 10% of the way to Elite to earn a single Cutter rebuy. (Admittedly, with only 4 internals, the Sidewinder isn't the best for endurance exploration, but if it has the endurance to reach Beagle Point how much more does it need?)
Mining: The Sidewinder is utterly terrible at this - nevertheless, with a mining laser, a class 1 refinery, class 1 prospector and class 1 collector, it can fill its 4t cargo hold with Painite in perhaps 15 minutes and earn tens of rebuys.
The Cutter can certainly mine a lot faster but even at maximum speed it might get a single rebuy in an hour.
Missions: The best paying missions might allow a Cutter pilot to get one or two rebuys an hour if they go all out for money in the right place.
Stacking a bunch of data courier missions lets a Sidewinder pilot get multiple rebuys in a single 10-minute trip - or a single surface scan mission can pay over 3 million credits which at Sidewinder level is basically unlimited rebuys.
(And the Sidewinder with its superior turn rate and smaller landing footprint is arguably better at surface scans than the Cutter is)
The problem, I think, is that the ship and module prices go up exponentially while the ship capabilities go up at best linearly with that (and sometimes are flat or go down) ... so anything balanced around costs - be that earning rates or rebuy - just doesn't work.