Why do you bruise after getting blood work done?

I’ve always had tiny veins that hate giving blood. Almost every time I’ve had blood work done in my lifetime it’s been an issue for me. They have to use the tiniest butterfly needle, half the time they poke me and no blood comes out and then they have to do it again, sometimes a bit comes out then stops and they have to do it again somewhere else. Also about half the time I bruise really badly and the other half I’m fine. Well the other day like maybe a week ago now I had to get blood work done so I went to a lab and the nurse poked me, the vein didn’t have any blood because I’m half vampire or something 🙄😂 so she had to do it again somewhere else and was then successful but afterwards I bruised like crazy all half way down my arm. I’m actually still severely bruised even today, black and blue and purple up and down my arm. One thing I noticed was when I got there I told her about the butterfly needle that they always have to use and I saw her get out a normal needle. I could tell because I’ve seen the butterfly one a million times and this one was for sure bigger and not the same, but I didn’t say anything because I knew they had to take five vials from me and the butterfly one goes so slow so i figured maybe she was trying it out with the bigger one to get it done faster. Now today I had to get blood work done at a different lab and again a different nurse I had had to poke me twice because the first vein didn’t bleed but she listened to me and actually used the butterfly needle and I had absolutely no bruising. Now I’m curious why does one bruise from blood work? Is it random? Is it based on gentleness of the nurses? Can needle size impact it? Does anyone know?