Coincidentally, we were discussing this at karate, with several people of the opinion that some people bruised more easily than others, ad this did not appear to be connected with the number of fights people took part in or their ability at fighting and defending against attack.
Bruising is basically damage to small blood vessels and leakage of blood. I could guess at possible factors - eg skin tone making minor bruises less visible, maybe more fat has a cushioning effect? Obviously technique does have an influence in caving - the more you bash bits of you against bits of rock, the more you will bruise.
From a random web site:
"If you bruise easily it may simply imply that your body is especially sensitive to capillary leakage. Other causes of easy bruising may include vitamin deficiency, aging, platelet disorders, kidney disease, leukemia, cirrhosis of the liver, side effects to medication (especially aspirin and warfarin) and Marfan syndrome...
Bruising easily is not always the sign of a serious disorder. In fact, it usually just means you are more prone than others to the phenomenon. Sometimes, it suggests a slight abnormality of the blood vessels or a deficiency in blood clotting. Also, if you are fair skinned, bruises tend to be more prominent and even the slightest bruising is visible.
Certain nutritional factors can also increase your tendency to bruise. For instance, excess use of ginkgo biloba, fish oil, garlic, ginger and vitamin E can lead to excess bruising. A deficiency in vitamin K, vitamin C and flavonoids can lead you to bruise easily. Vitamin C helps bruising and flavonoids are usually recommended along with vitamin C, since they increase the effectiveness of the vitamin..."