To fully appreciate what's going on here, here's some basic septic tank anatomy and physiology.
It's not the size of the septic tank that is causing your problems. It's how saturated or compacted or coated your leach field is. In the tank, solids (poop and toilet paper) sink. Fats and grease float to the top. In the middle is cleanish-dirty water. Your tank has a baffle, which is an elbow of pipe that allows the middle water to evacuate the tank, but not the solids or fats, into the leachfield where the water filters through layers of sand and gravel. If your leachfield is swamped, coated in grease or fabric softener and isn't allowing water to drain through the sand, you have a problem. If your baffle is broken or corroded, you also have a problem. Solids and fats will clog up your leachfield. So do tree roots, driving over the field (compacting the sand), or even a broken leachfield pipe. A healthy tank operates 'full'. Frequent pumping lowers the fat level to the height of the baffle and leachfield outlet, which will ruin the leachfield in no time at all. So don't worry if they say the tank is 'full' unless the level of solids is reaching the height of the baffle. It should always look full. Unless of course it's backing up into your yard or home. That shows a probl with the leachfield or baffle.
If the leachfield isn't swamped with water, it may be clogged. Call RotoRooter or a similar company that has high pressure hoses or snakes they feed up the leachfield and auger out clogs or roots. We just did that last week and it's buying us some time.
I've pm'ed you with graywater suggestions, and am battling the same situation in the same geographic area.
If you can, upgrading to a front loader is the most water and energy efficient change you can make. Pumping the wash water to a seldom used part of your yard or drainage ditch is illegal in nh, but rarely checked up on. Run the washer hose out a window or through pipes outside. Be sure to use nontoxic soaps with no whiteners, bleach, fabric softeners or phosphates to reduce harm to the enviroment. You may decide to use diapers that wash thoroughly with less water like flats and prefolds and microfiber soakers versus AIOs and thick fitteds.
Septic guy is probably just covering his bases. A top loader uses like 40 gal/wash, right? I've had to use the laundromat recently because of the flooding. Actually I liked it. It was a nice, quiet break away from the chaos of being cooped up at home during the floods.
I left the kids with dh.