Your paddock boots may be too big, but it is unlikely that that is the reason for your feet coming out of the stirrup at the canter. It might make it a little harder for you to be really stable on the ball of your feet, though.
Usually, feet coming out of the stirrup at the canter is due to gripping up. You don't feel secure, so your grip onto the horse with your calf and knee and your heel comes up and back.
Look at the inside of your paddock boot, or half chap if that is covering your boot. Do you see an area where there is rubbbing indicated on it? Like a whiter area? If you don't ride a lot , you won't see this. If you are gripping a lot, then you tend to see this on the inside of the boot. I know that I tend to grip more with the right leg, and when I was more of a beginner, it used to cause me to lose my stirrup at the canter, every time. Over time I learned how to not grip and to keep some weight going down into the stirrup and now I don't lose them except in really unusual circumstances (like horse bucking).
So, just try to relax more at the canter, let your weight drip down into the heels. The boots might be too big, but they are not the root problem, IMO.