use a lowe one – meant for cameras – and used it when I originally mountain biked – 20 years ago. It fits gel bars, drink (small), tube, repair kit, phone, car keys, tools, tyre levers – and is still pretty small. Slips round to front for access, rear for rising. Has wide straps so doesn’t pull to much, but it probably compresses my waist more than a camelbak or other would. essentially, it’s fine.
However, I’m going to get a Camelbak – will take a bit more for days in the hills, easier to drink from on the go (and I need to do that more) – but I’m getting a Charge LR – has a lumbar reservoir so the water and weight is low down around the waist – it seems much more sensible for mountain biking than carrying it higher on the back, and I’m used to the waist weight.