Softimage Xsi: a question about xsi's preset biped setting~~

i use biped guide commnad create a biped guide, then i scaled up my biped guide's null (local at the foot area), then i use rig from biped guide to get a biped bone from biped guide, but the new biped bone 's volume automatic scaled too big, why? i have try it alot time, but is still not have some change, i use xsi 3.5.

who know why about this? please give me some help~~~