We are traveling to Germany in May for 10 days and I'm trying to replace my wife's huge 29" suitcase with something more reasonable for the trip.

I bought a Victorinox 24" monotraveler from REI outlet because the volume was listed as 3900 to 4600 cubic inches. But the suitcase dimensions are 14x24x9 which comes out to 3024 cubic inches. I noticed a lot of the Victorinox volumes seem large for the suitcase dimensions where as Eagle Creek seems to match better.

Is there something I'm missing or don't understand? Anyone else notice this too?

Did you include the amount of stuff you can put into the zippered compartments as well?

Not really but I thought about it. The only external compartment on this particular bag runs about 3/4 the length of the suitcase but doesn't expand out much if any. If I do some quick math I get another 300 cubic inches at most.

I'll bet that's part of it, or most of it. If it has outside pockets on most of it, as many suitcases do, that will add on another 1.5-2 inches thickness, and for a 14x24 size, that's about another 500 or so cubic inches right there.

I buy suitcases based on looking at them and seeing their features. That's a good size to use for a replacement, a 24", not too big and not too small.

Is that model expandable? I have another brand that has a zipper around the side; when I undo the zipper, it becomes 2 or 3 inches thicker, for those times when I've extra things to carry.

Yes it's expandable hence the volume range of 3900 to 4600 cubic inches.

I should have her do a test pack this weekend. I need to do the same too since my suitcase is a 21".