I'm saving up for an All American pressure canner. The 21.5 quart holds 7 quart jars, while the 30 quart holds 14. There will be times I won't have 14 jars to put up, so is there any problem running a great big one like that half full? Any other considerations? My thinking is if I'm spending the money, it's only about $70 more for twice the capacity, but I've never used one before so I'm not sure if there's something I'm missing.