Well, if I understood him right, bunch beans grow in tighter clusters and are smaller and rounder, while running beans climb along poles and get larger and flatter. They're not different species, just different cultivars. According to GardenLad at the Heirloom Plants & Garden Forum, there are similar distinctions among green beans.
In some places, though, if you ask if it's a pole bean they'll look at you strange, because---particularly in the mountains of the South, and in the Ozarks, they differentiate them as stick and bunch beans, rather than as pole and bush---which, btw, are called "dwarf" in England and some parts of North America.From this I conclude that the more space you give a bean to grow, the bigger it'll get.
According to Japan's NIAS Genebank, the same is true of pole and bunch cultivars of Phaseolus lunatus L., a bean of many disguises and aliases.
Pole type cultivar and wild form of P. lunatus are twining, perennial herbs, 2-4m tall, with enlarged rootstock (Purseglove, 1974). Annual and small bush forms, 30-90cm high, have been developed in cultivation.In my experience of Southern usage, butter beans are the smaller, sweeter, greener varieties that are eaten as vegetables, while lima beans are the larger, starchier, whiter varieties that are more often found in soup. (Ochef seems to have it precisely backwards.)
The University of Melbourne has a very useful, multilingual compilation of names for different varieties of the bean genus Phaseolus. Phaseolus lunatus L. is divided into three broad groups: Lunatus, the large limas of Andean origin; Sieva, the small-seeded limas of Mesoamerican origin; and Potato, the round-seeded, Caribbean limas. The principal Japanese term for limas is ライマメ raimame, which seems to have been formed by haplology from ライママメ raima mame 'lima bean'.