Vaginas aren't a static size - they can expand and stretch to accommodate things like tampons. So it's not likely that the size of your vagina is an issue. However, if you aren't bleeding much, then tampons can feel pretty uncomfortable; there are smaller tampons made for lighter bleeding but some folks find pads or menstrual cups more comfortable for very light bleeding. Tampons are designed to absorb fluid, and if you aren't bleeding much then they can feel very dry and uncomfortable. They're perfectly fine to use with lighter bleeding, you just want to use one designed for the amount of bleeding that you're having. (So don't use a super-absorbent one on a day when you aren't bleeding much, for example.)
If you're tense or anxious about using tampons, that can cause some discomfort too: it might be that your muscles are tensing and making it difficult to insert a tampon easily. Sometimes the angle can make a difference as well, because the vagina doesn't actually go straight up into the body, it's angled towards the back. So if you're inserting it straight up or trying to, then that can cause some discomfort as well.
It might also be that tampons just aren't the best fit for you right now, and that's okay. Not everyone likes them or finds them comfortable, and you don't have to use them if that's the case.
"Where there is power, there is resistance." -Michel Foucault