I expect that with Ronseal you get what it 'says on the tin' lol i.e. they state 275 grams which is the weight of the filler you will be getting. With Isopon they are quoting a volume (of the tin). 250 grams of Isopon wouldn't be the same as a volume of 250 ml hence why it looks less. Isopon would be better to quote the weight of the product as per Ronseal. It is bit bizarre of Isopon really as they are using weight to check the fill levels. I am sure both are very good fillers.