Chambers Dictionary lists 'shed-load' under the heading where 'shed' refers to the store-room/outhouse meaning rather than under the second meaning, where 'shed' refers to things dropping off. The two words, though superficially identical, come from totally different sources.
A shed, of course, needn't just be a little thing in a garden; there are, for example, massive naval structures referred to as 'submarine sheds'. So, a shed-load means a quantity notionally large enough to fill such a place.