Speed debate again - 3 ft. versus 2 ft.

If you scroll down in the link below, you will find a photo of the engine crew cooking their eggs for lunch in the blow down vent. In the same photo a speedometer is in evidence.

https://sites.google.com/site/soulorailway/home/system-3-1/part-7-avontuur-to-assegaaibos

This page is about the 2 ft. narrow gauge of the SAR. The NG-15 2-8-2s and the NG-16 2-6-2+2-6-2 Garratts.

If you read the content below, it is noted the NG-15s could hit speeds of up to 40 mph.

Taken with a grain of salt, I will say more likely 30-35 mph.

How are these 2-8-2's (2 ft.) able to hit these speeds and not the 3 ft. 2-8-2's?

I would be interested in learning the physics and engineering differences that allowed for these differences in speed.