I know it's 2 discs, the first is 50GB and contains Episodes 1 to 8, the second is 25GB and contains 9 to 12. Normally FUNi puts 9 episodes on the first disc, but the 8 here is the same run-time. Episode 1 of Re:Zero is 50 minutes long, so it still is mastered the same way.
Now, with 8 or 9 episodes per disc, you can still get a great looking disc, but it is more taxing on the compressionist. When aiming for a lower bitrate, it is more time consuming to produce a good result. A good example is Sentai's
SNAFU releases. These are practically
flawless discs with zero banding, aliasing, macroblocking and other artifacts. SNAFU S2 might be Sentai's
best release in terms of PQ- it's just that good. So, it is possible to make good releases with this limited space.
Lately, FUNi has started to move away from that format when dealing with special cases.
The Vision of Escaflowne had extended scenes for the Laserdisc release that were never dubbed (more about it in the link in my signature), so the show was re-dubbed and a separate video track for 7 episodes had to be created. So, the show is formatted in a 7-7-6 config for set 1 (the first 7 episodes are done twice, for the broadcast and extended versions) and 7-6 for set 2. This results in perhaps FUNi's best release in terms of PQ.
Code Geass is another interesting example. Ignoring the audio issue in R2, it is laid out in a 7-7-7-4 format due to 3 audio tracks and numerous extras for each season.
Re:Zero is up there with Steins;Gate and Psycho Pass as one of the worst releases by the company, and there's no excuse. They knew it would sell, so why not make a good product that everyone will enjoy? Other than rushing it out the door, I have no idea...