The 2020 Acura RDX is considered a compact luxury SUV, but there's actually very little that's compact about it. When lined up against its competitors, you can see that it enjoys a clear dimensional advantage. In fact, the RDX is pretty close to the Lexus RX, with an identical rear legroom figure and considerably more cargo space with the back seat raised. Obviously, I'll be addressing the latter here.
On paper, Acura provides several cargo numbers calling out specific SAE measurement standards for each one. Basically, it has 31.1 cubic-feet behind its back seat when you include its underfloor storage and 29.5 cubic-feet when you don't. Even that smaller number is superior to everything in the segment save the Volvo XC60 (29.7), while the Lexus RX apparently has a seems-way-too-small 18.4.
Let's see what that size advantage means.
It's obvious that this is a comparatively deep and wide space with plenty of air above the back seat line for bulkier items. Note that the cargo cover was not present in this test vehicle, so I couldn't test capacity with it in place.
As in every luggage test, I use two midsize roller suitcases that would need to be checked in at the airport (26 inches long, 16 wide, 11 deep), two roll-aboard suitcases that just barely fit in the overhead (24L x 15W x 10D), and one smaller roll-aboard that fits easily (23L x 15W x 10D). I also include my wife's fancy overnight bag just to spruce things up a bit (21L x 12W x 12D).
There you go, all the bags fit easily. This is pretty much identical to the Lexus RX (below left) and a much easier fit than the Mercedes GLC, which is one of the only other compact luxury models I've tested. Both of these examples are more capacious than their specs would indicate.
However, this is not the end of the RDX story. Remember when I mentioned that it has 31.1 cubic-feet when including its underfloor storage? Well, that might actually be underselling the extra space gained.
First, I like that the floor flips back and rests flat so you can easily use the extra space. You can also fully remove the floor piece to gain access to the aft bin, which is deeper but oddly shaped.
Here are my four largest bags standing up, with sufficient rear visibility maintained, and you still have the full-width section of the cargo area including the underfloor bin. What can you fit there?
How about a 38-quart cooler and the remaining two bags stacked haphazardly around it. I also could've fit a Graco playpen plus some smaller duffel bags.
Essentially, when you use the under-floor storage, the RDX does indeed have far more cargo space than the competition. Now, the Lexus RX has it's own cargo-expanding trick, a sliding back seat, but using it obviously comes at the expense of your passengers' legs. This comes at the expense of nothing. A spare tire is still present in some RDX's, though not this A-Spec trim.
In case you're comparing the RDX within the Honda compact SUV family, here is what the 39.2-cubic-foot Honda CR-V can manage. Basically, it can do the same as the RDX, but without the need for secret underfloor compartments. It also, obviously, makes for much neater, easier loading.
Now, here is the RDX's full, seats-down cargo capacity. The floor is reasonably flat.