I bet switching to LZMA would have saved even more. LZMA beats Brotli nearly every time. zStandard would likely have worked better as well. Brotli is very slow to compress.
I'm sure you can use those results to argue that LZMA is superior in some way (e.g. compression speed) but it definitely isn't clear cut superior in other important ways (compressed size and decompression speed are inferior).
I can see why, given those results, that they would use Brotli over LZMA.
Apps are usually in the order of 40-50 MB. From a back of the hand calculation, the a 50 MB file when compressed with Brotli is 14.5 MB and LZMA is 17.5 MB. The difference of 3 MB translates to an additional 8 seconds of data transfer in a 3.1 Mbps 3G connection (an average one).
This [1] states that a LZMA has a decompression speed of 70 MB/s, which is about 0.7 seconds. The 334 MB/s speed of Brotli does the same in about 0.15 seconds. So the additional overhead of LZMA (compared to Brotli) in decompression is just 0.65 seconds.
Given these orders of magnitudes, I think optimizing for compression ratio is a much better option than decompression speed.
I agree that LZMA beats Brotli in compression in the majority of cases (not by much), zStandard does not however.
The thing that makes Brotli attractive though, is that it has high compression (again, very close to LZMA, sometimes even better) while decompressing MUCH faster than LZMA.
The big downside is that it is very slow in compressing, which makes it mainly suitable for 'compress once, decompress MANY times' type data.
