Tip a higher percent when kids aren't with you since cost is lower?

Have a wife and 3 kids. Usually we go out to eat and it's anywhere from $60 - $100 for all of us to eat as they're all teenagers. Tip the normal 20% so $12 - $20 for decent service.

Went out to eat with just the wife and the bill was only $33 so tipped $12 because I felt like I was getting such a bargain compared to the normal price paid compared to when all of the kids are with us. Obviously we didn't go anywhere fancy for that low of an amount. If we went to an upscale place where the price for 2 was $100+ then the tip would've been the normal 20%.