It depends on the product.. it does take more than I thought to get weighed down.. It happens more when my hair is straight, my natural oils will kill all chances of volume by day 3. With it curly its a different case. I think you are partially right.
I saturate my hair with coconut oil and let it set overnight and its only slightly greasy when I wake up. By the end of the day my hair sucks it up completely!
Lately to seal in more moisture ive been running butters through the length of my dry hair, it does not weigh down my fro at all, unless I go overboard. I never thought id be able to do that before!