The condition of the road network in Spain was a large factor in using the 4- and 8-pounders as the field pieces of choice. Horses are always an issue, especially when the wars had been going on for sixteen years when the French went into the Peninsula. Further, Spain is not noted, I believe, as horse country.
Capability and weight of field pieces is always an issue. That is still an ongoing argument, sometimes to the point of idiocy (here I am referring to a modern argument in the Marine Corps as to which field piece is more capable and deployable, not to anything or anyone on the forum). The bottom line, though, on which artillery piece is employed can many times be reduced to British artilleryman General Phillips simple argument in 1777: 'Where a goat can go a man can go; where a man can go he can drag a gun.'
Artillerymen, as you know, generally make do and get the job done anyways.