All martial arts confrontations can be broken down on two dimensions: mental/technical and physical. You can think of it as having two weights each to put on a scale, and whoever has the higher combined weight wins. So you can have a slight mental/technical deficit in comparison to your opponent but a huge physical edge (maybe you outweigh him by 50 lbs) and you'll still win. Or you might be much smaller or older or weaker but have a significant mental/technical edge and be able to beat him despite your physical disadvantage. This is what happens when athletic white belts roll with older black belts. There are also diminishing returns as both your skill and physicality increase in comparison to your opponent, which is to say that generally if you're both high on one dimension and low on the other the match will be decided by who has the edge in the dimension you're both weaker on. So two very athletic people with middling technique fight, it will probably be decided by who has the better technique rather than who has the better physicality. Likewise, if two guys have great technique but aren't very athletic even if one guy is slightly more technical but less athletic he'll probably lose most of the time. In your case OP you're deficient on both dimensions as a smaller white belt against bigger blue belts, so you're just going to have a rough time. Honestly, it wasn't until I was late blue/early purple that I was able to easily handle bigger, stronger white belts, and even as a late purple I had guys (a 330, 6'8" blue belt comes to mind) who I couldn't do a whole lot with. Technique can only compensate for so much difference in physicality, though the converse is also true.