The Siemens 6ES7953-8LG00-0AA0 is a SIMATIC S7 Micro Memory Card — a 128 kbyte Flash-EPROM module that stores the user program and data for S7-300, C7, and ET 200S IM151 CPUs. It runs on 3.3 V NFlash technology, so it's the program carrier, not a RAM expansion. That 128 kbyte capacity is the hard limit for the application code and configuration data; if your project compiles over that, you step up to the next card size.
Flash-EPROM means the card retains the program without battery backup — power it down, pull it, plug it into another CPU of the same family, and the logic is still there. That's useful for cloning a panel or swapping a failed CPU without a fresh download. The 128 kbyte figure is the usable memory for blocks, DBs, and system data; the OS itself lives in the CPU's internal ROM, so this card is purely the user-program space.