Foam never seems to expand to the advertised amount . . . probably because it is not hot enough when it is activated and poured. So, you buy enough and as you pour it does not seem to be enough. If you read the fine print on these 2-part foams, it indicates that the working temperatures is spec'd at 80 degrees.
My advice is to try to get the 2 liquids warmed up ahead of time and also the boat hull and chambers, so that you get the most expansion possible.
When I foamed in the fuel tank on my Formula 330, I pre-heated the liquid A & B parts to about 85 F and the hull chamber to about 75 F . . . It seemed to work out great and I got lots of expansion. The ambient temperature that day was about 65 F, which would have resulted in about a 25% loss of foam expansion.