Buttload is used to express a vague but large quantity; it is often assumed to be a meaningless word (stemming from butt in the sense of buttocks) of the sort we’re so fond of coining based on vulgar words (cf. shitload, or the myriad words and phrases based on em>fuck). A buttload is, as it turns out, a more or less quantifiable measure, in no way related to the buttocks or their capacity.
The butt referred to here is a heavy, two-wheeled cart, drawn by oxen or horses (for example, a dung-butt is a cart used to haul manure). A butt-load is the quantity that your average butt can hold. In the earliest written description of a butt-load (1796), this is said to be “about six seams.” A seam is “a pack-horse load,” the exact quantity originally depending on both the locale and the item being measured. A seam of glass, for example, was 120 pounds; a seam of apples was 9 pecks. Later, a seam was more or less standardized at 3 hundredweight of hay or manure, or 2 hundredweight of straw. So a buttload of manure would be roughly 2,016 pounds (a hundredweight is about 112 pounds; 3 hundredweight makes a seam; 6 seams is a buttload).
Butt, by the way, is not a shortened form of buttock. Rather, as near as anyone can tell, buttock is formed from butt by addition of the suffix -ock, which is used to form diminutives. This is the same -ock found in words such as hillock (a small hill), tussock (originally a tuft of hair), haddock, and bullock (originally a young bull, later a castrated bull; bullock comes from the Old English bulluc—the -uc suffix is a variant form of -ock).