E_merlin, Another contributer to a bicycle's stability is the wheelbase. The shorter wheelbase became, the slacker the HT angle became (to increase trail) to prevent a nervous riding bicycle.

Another reason for the slacker headtube angle on smaller frames is to simply fit a commonly used size wheel/tire that doesn't interfere with the downtube or the rider's toes. Sometimes the smallest of frames were accomodated with a smaller front wheel, which would cause another trail calculation.

Brad