Yeah, colubrids and pythons definitely have starkly different figures. Colubrids are GENERALLY more active hunters, vs BPs which have GENERALLY shown more ambush hunting disposition.
No scientist or anything, but probably the weight differences are indicative of their natural habitat as well, with there being some research that says that BPs mostly only hunt during rainy season in the sub-Sahara, which means (if true and iirc) they have extended fasting periods and thus need to store their fat vs colubrids who are used to hunting every few days or more often if need be.
Edit: Having a scale with tare helps. Turn it on, put a bowl big enough for the snake on it, press tare, put snake in bowl. Tare subtracts/negates the bowl's weight and the bowl keeps the snake in one spot.