Found the book I was looking for: R.Peele (ed) 'Mining Engineers' Handbook', New York, 1927. Unfortunately it did not give loads for mules and horses. Instead it limited itself to comparing their relative advantages for underground rail haulage.
Going off-topic, its comparison (p.1024) stated:
'Horses vs mules. Mules avearge smaller, require less headroom, endure heat and neglect better, and are less liable to foot lameness than horses. They will eat roughages which horses refuse, but are less apt to overeat, and hence less subject to colic or founder. Horses are heavier, better built draft animals than mules, average more reliable, haul larger loads, and require little or no more fooed per 100lb live wt than mules on similar work. They are more spirited than mules, and a nervous horse is apt to rear and injure his head against roof. Mules are more generally used underground than horses but, where haulage ways are large enough and the duty sufficient for heavy horses, the advantages of mules are debatable, and where there is headroom many coal operators now use large horses in preference to mules.'
An unromantic view of horses and mules!
Looking afresh at the image that started this thread, Spitfire is perfectly correct in stating that there are some major components that were certainly not mule-packable - e.g. the solid cast flywheel which looks to be about 5 foot diam.