Hi Tijmen,
I came across this the other day and found a number of articles on it, this one explains it best:
https://lemire.me/blog/2019/01/30/what-is-the-space-overhead-of-base64-encoding/
It’s not Mendix specific, but the idea is probably the same. As for practical usage, base64 strings in your DB use DB space and processing instead of disk space / regular cpu processing, which may be more scarce depending on your Mendix plan.
Hope this is enough info for you!
Regards, Thijs.