Appearing average size is completely fine. Stones that appear "large " for their carat weight are typically more shallow than those that appear "average". As long as the stone scores under 2 on the HCA, I wouldn't worry about it. The only thing to avoid (in my opinion) is stones that appear "small" because that likely means they are cut deep and will look smaller than their ct weight
Average size is great. If it faces up “bigger” you run the risk of the diamond being cut too shallow and the optical properties won’t be ideal.