Simply put smaller the bubble smaller the gradient of buoyant forces (they act on all sides of an object and not just from below). So basically bigger bubbles accelerate faster than the smaller ones. Smaller bubbles don't rise slower they have a more uniform ascent. Now when you are thinking about a bubble's terminal velocity, it depends on friction which depends on again pressure and shape of the bubble. So now there will be a slowly depleting friction and a slowly depleting net buoyant force. Now you will need to do some calculations to find out how much these are depleting with time. Basically depending on the liquid if buoyant force depletes faster there will be a terminal velocity and if friction depletes faster then it will keep accelerating. But they both reach zero at the end simultaneously so it's about the curve.