It’s not the size of the engine, it’s the compression vs blade length (leverage).
When I flew it, John Jordan’s Stearman G AROY did not have a starter motor, so you had to hand swing the R985 radial engine, but the compressions are low and the prop is big.
Philip Meeson was always warmly greeted when he arrived at the Tiger Club in his Pitts.
Then, when it came for him to leave, there was suddenly no-one around except stupid me!
The Pitts’s engine had high compressions, and she was a b*tch to start!
Then there was the day there were plenty of prop capable people in front of the hangar while out on the grass the Beagle Terrier wouldn’t start.
The pilot opened the throttle and got out to blow it out himself.
At full throttle the Terrier tore a Turbulent apart, threw the bits at a Beta, and then chomped into the port wing of the Mew Gull sitting at the fuel pump.
though there's the 1 out of 10 who have a well oiled routine and prefer to do it themselves.
I have run across this recently at a Fly-In, I know the pilot does not particularly like me, but I still offered to swing his propeller.
As above he prefers to do it himself.
What I usually do in such a stituation is walk away, but not far, I tend to stand in front of the tailplane.
Safety is an attitude, and my attitude is not to walk away from someone doing something that may be risky, rather be in a position to prevent an accident or reduce risk.
The aeroplane owner does have his well oiled method of starting his aeroplane. So the other side of the coin is to not get in the way of this, disturb the sequence, cause him to miss a step in his procedure.