Ok, I'm italian and my english is not perfect, but I'll try to explain myself.
Aries has two different "partitions":
1) Memory Partitions: Mi2s at the beginning has memory for the apps (the famous 3.7 Gb), and memory for Data (such as documents, multimedial file). In the 5.5.15 version of Miui, Xiaomi decided to merge this two MEMORY partitions in one memory: you should have had to flash the 5.5.15 Miui zip via Mi PC Suite and fastboot. Unfortunately this zip is no longer avaiable, it's too old, and flash the most recent update via Mi PC Suite will not work. So the only thing you can do is change the partition's table, and bring some Gb from Data partition to Apps partition.
2) System partitions: Mi2s, such as some other Xiaomi's devices, has two system: system 1 and system 2. When you flash a zip from Updater app, it will be flashed into the system not in use (for example if you are in system 2 with Miui 5.11.12, updater will flash the update 5.11.19 into system 1, so if something goes wrong, you can switch into system 2 via recovery and return to Miui 5.11.12). After the Miui 5.11.19/20 Update, this SYSTEM partitions will be merged, so via recovery you'll be not longer able to switch from system 1 to system 2 and vice versa, because there'll be only one unique system!
Hope you understand