The larger the box, the slower the rate of change. It's a double- edged sword, though, because it takes longer to heat up as well.
When we were using our electric heater that is mounted on the side of our supply tank we would basically would have to plug it in over night to have the water hot enoug for the next day. It would generally drop around 10-20 degrees in 1hr in our 65 gallon tank Long story short, electric heater went out and we ended up going with a LG HT-2. One of the best decisions we've made for the company. Ivan