There's a nice item here about the origins of the mini-rigs:

http://theswca.com/images-speci/mini-rigs/mini-rigs.htm

Basically it started with bored workers at Kenner playing in the spare parts bin, but then they thought it would be a good idea to make small vehicles that children could buy with their pocket money as the larger ones were rather pricey.

None of the designs actually appeared in the films though sometimes you get items like the Imperial Tripod gun being classed as a mini-rig because it was in the same price range so in a sense they were (with Splinter Of The Mind's Eye and the Marvel Comics) the beginning of the EU.