How have you managed the 'all else being equal' part?

By eating less for a couple of months without any other major change of habits.

Most things that cause you to have more stamina* also cause you to be slimmer.

(provided slimmer is defined in terms of fat mass alone and not total mass: muscle weighs a lot, and...).
Sure, eating a lot by itself also makes me less energetic for a while (probably due to digestion requiring energy), but I'd expect that to be a short-term effect only.