There are lots of good principles for getting through life. Whole religions and moral traditions and Web sites are there to tell you the right and wrong ways , both general and specific, to do things. Here's mine:
Never try to carry two cardboard boxes of different size.
This applies whether the boxes are full or empty, whether you're carrying crisp ones out of the new-box store or lugging floppy old ones down to the recyclables dumpster. Different-sized boxes will slide around, and because their edges don't line up, you will keep having to stop and try hitch up your burden to get it back under control, if you don't drop one of the boxes outright.
There are no exceptions. I have learned this. Since the day I recognized the rule, I have never regretted obeying it, and I have always regretted breaking it. If the boxes are all one size, carry as many as you can lift. Two sizes, two trips.