I'm beginning to think that all car backseats are the same size.

Not that anyone else on here does this, but I'm window shopping on FB Marketplace, and every once in a while I'll see a car that tickles my pickle that I get curious about. One of the things that is important for me is to be able to put my kiddos in the backseat. One in a forward facing seat, one in a baby bucket. So before I get all hot and bothered, I look at rear leg room. I realized the other day, that either every automaker is lying about these measurements, or I'm really missing something about how the physical packaging makes a car larger or smaller in the back seat. 

I have a gen 1 MS3, and C&D lists the rear legroom at 36.2". I feel it's pretty tight, but doable. C&D has the MS6 at 36.5", okay...

They have a 2008 Outback at 33.9" which is from what I can tell an objectively larger car. 

Edmunds has our 2000 LX470 at 34.3" and our gen 2 Sienna at 39.6". 

So between these options, there is a 5.7" spread. But when we pick a car to road trip in, it's the LX as there is more space in the backseat with both front seats in a normal driving position. The Sienna is massive, with sliding back seats, so it's not a rightfully fair comparison, but it's an interesting datapoint. 

I was reading that rear leg room measurements are taken with the front seat 1 meter from the steering wheel. I am wondering how these numbers can be so close if I don't find my arms growing longer or shorter as I move from vehicle to vehicle. 

What am I missing? Is the rear passenger compartment of my MS3 larger than I think? Are these numbers just total bologna? 

I can stand (hunched over) in the backseat area of my LX to buckle/unbuckle my toddler, it's basically a dance floor. With a baby bucket in that car behind the drivers seat, there are multiple inches of clearance between the car seat and the driver's seat. With the MS3, they contact each other. Yet the MS3 is supposed to have more room. 

Besides physically putting my body and a car seat into a vehicle, is there another way to interpret this data? What am I missing?

Just out of curiosity I checked the rear seat room for my wife's 2006 Scion xB.......38"........it feel huge back there with either of us in the driver's seat.

It's because you are looking at leg room.  Rear facing kid seats hit up near the top of the seat and forward ones put the kids legs almost straight out and near the middle/top of the back.  Depending on how the front seats are shaped/thickness/reclined that unmeasured volume can disappear real fast.

My kids couldn't fit in my GT500 because the seat was too deep and therefore their legs either stuck out, hitting the front seat, or they had to slouch so their knees cleared the seat bottom.  I fit in them better.  Leg room was manageable when the leg is vertical.

Those "new" plane double decker seats/meme is a good visual.  They have more leg room, and the seating for a full size adult works, but a kid seat in one would be a non starter.

I know I say this a lot.... but check the Koreans. Their rear seat room on their midsize/compact cars is extremely generous. To the point I have no problems hopping in the back seat behind the drivers seat set up for long distance comfort. The Subcompacts (Accent/Rio) are still small, but the compact (elantra/forte) and midsize (Optima-k5/Sonata) are good.

Rear seat "volume" makes a little more sense than "depth" because of that silly front seat in the way.  You know, where the chauffeur sits 

This is why there are four door coupes and two door sedans, the term is relative to rear seat volume...

First, while there is an SAE recommended measurement method, there is not a mandated one. Second, rear legroom is also not a linear measurement from the back seat to the front seat. If I'm not mistaken, it's a diagonal measurement from the hip center to the ankle center (+10 inches). See L51. Also, I believe the rear legroom is recommended to be taken with the front seat legroom (L53) at 40 inches. So there are a lot of variables that affect how any given measurement 'feels' from one car relative to another.