From what I can tell, the avatar Physics coinsidering it comes from Havok, is actually a mix of 5 things. Maybe 4, but thats only if the 5th isn't implimented as I think it is.
1: Breast Physic Setting - Note, I use Nirans, his viewer does NOT use sliders, and instead uses virbalized settings None, Very Low, Low, Medium, High, and Extreme. I use Extreme, and the bounce does not bother me. It should be noted if this setting is like havok enabled games, it is the setting for PHYSICS REFRESH RATE. Not how bouncy this are.
2: Breast Size, This also goes for Butt Size, and Muscle/Fat Ratio, because the SL skeletons for Havok to create boucne require weight on parts that bounce, cleavage, butt, and so on. Adding extra size, streches these weight locations influencing more bounce. IE - More Size, More Jiggle.
3: Breast Gravity((THIS IS QUESTIONABLE)) Now reason I say this is questionable, is because Gravity makes your breasts droop w/o Physics Enabled, or even attached to your avatar, so it's my thought Breast Gravity, adds extra weight to the skeleton map on the avatar, but I could be wrong, and it may just be cosmetic.
4: Extremity of your Physics Layer enabled on your avatar, I only buy Moddable Physics layers for this reason, heck I only have one, so not like I need anymore, These Sliders effect how powerful your bounce is.
5, Last but not least, the viewer you use((Yes, thats right, Viewer has effects on it)) Now the main reason for this, is some allow you to force Physics on your avatar, rather then using a Physics Layer, simalure to how Frestorm comes with an AO((Dunno why, because Oricle and Zhao AO's without anims inside are free)) These settings can alter how Havok effects things on others, as my friend used the Physics Slider on Emrald after Havok was implimented and her **bleep** were doing a disco.