system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do phones have a limit of how much expandable storage they have?

As in, say there's a phone that has 32 gig and can be expanded up to 64 with an external SD card. Why wouldn't it be able to go up to 128 or something with a bigger card? What's limiting that?
gpt
Different standards. 

There are three different types of SD formats, these also apply to MicroSD. 

SD: Up to 2 GB

SDHC: Up to 32 GB

SDXC: Up to 2 TB, although cards of that size don't exist yet. People have only been able to make 256 GB MicroSD cards, and those are still very expensive. 

So when a phone manufacturer says it can take MicroSD cards up to 128 GB, it means it can take SDXC cards, but they don't want to be misleading so they say 128 GB instead of 2 TB.