My bad. I'm trying to standardise on original sizes of 1000x1000, but some aren't quite so big, especially a heap of legacy ones which are less than 300x300. I'm using 300x300 for the product image displayed on a product page and 100x100 for small images. I find it strange that the optimisation app is suggesting those small images generated by IH are 10% to 20% too big.