There's actually a perfectly reasonable answer.
We've found an interesting discrepancy with the 2021 BMW 3 Series. Ever since the 2019 model, when the 3 Series switched to the G20 generation, BMW has rated the trunk space at 17 cubic feet. That is a massive amount of space for a compact luxury sedan, especially considering that many "larger" midsize models have smaller trunks. At least, that is what the numbers suggest.
BMW's official website now cites the 2021 3 Series with 13 cubic feet of space, much lower than the previous 17 measurement. The 2021 3 Series is a complete carryover from last year's model except for a few engine updates, so what's the explanation for this seemingly random change?
As it turns out, the trunk didn't get any smaller; the unit of measurement simply changed.
BMW previously relied on the Society of Automotive Engineers (SAE International) standard to measure its trunk space. This system uses simulated luggage parcels, including blocks with handles to represent suitcases, standard golf bags, and smaller bags to measure remaining crevices.
BMW now uses the EPA measurement system, which calculates trunk space differently. It's worth noting that there is no single standard by which all automakers measure their trunks, so there are likely many discrepancies across the industry.
A BMW spokesperson iterated to CarBuzz that "we believe that the EPA trunk volume is the most appropriate figure to communicate for our vehicles in the US." For reference, the Audi A4 also uses an EPA measurement scale and is rated at 12 cubic feet. This is still less than what's offered in the 3 Series, but the difference doesn't appear to be as large now that both cars are measured the same way. Likewise, the Mercedes C-Class is no longer at such a disadvantage with 12.6 cubic feet of space.
The 3 Series still boasts one of the largest trunks in the compact sedan segment, though it is no longer class-leading by a wide margin. The Acura TLX and Infiniti Q50 each manage to outsize it, both boasting 13.5 cubic feet in their respective boots. In reality though, that extra half a cubic foot won't make a significant difference on luggage space, so the measurements are pretty inconsequential. Perhaps one grocery bag will have to sit in the back seat.