That's an extension on the standard POSIX behaviour. c is the only suffix supported by POSIX find -size. Without suffix, the unit is 512 byte blocks (formerly commonly known as sectors).

-size n: matches on files whose size rounded up to the next integer number of 512 byte units is exactly n. For instance, -size 2 selects files with sizes 513 to 1024 bytes.

-size -n: when that number is strictly less than n. So with -size -2, that's files with sizes 0 to 512 bytes

-size +n: when that number is strictly more than n. So with -size +2, that's files with size of 1025 bytes or more.

GNU find extended it with more suffixes (kmg...) but with the same semantic. Same happens in zsh with its L glob qualifier: *(LM-2) matches on files with size 0 to 1048576.

FreeBSD find also added suffix support in 2006, but while it honours the POSIX rule for -size n without suffix, with suffixes, it uses the more intuitive semantic where -size 10M is the same as -size 10485760c.

I agree the GNU behaviour is more confusing and unintuitive, but it's too late now to change the semantic. It would mean not being standard compliant for suffix-less numbers and it would break backward compatibility (though it's likely it probably would also fix some scripts which expect the more intuitive behaviour)

What could be done is introduce new syntax with a different semantic. For instance, we could have -size '<=10MiB' to match on files that  are less than 10MiB. Or we could have a new -apparent-size '-10M' with the more intuitive API. I'll let the GNU find maintainers comment on whether they would consider doing it.

Note that there's a similar problem with -atime/-mtime... where -mtime +n matches on files whose age rounded down to the next integer number of days is strictly greater than n.  So for instance -mtime +1 matches on files that are over 2 days old, not one, because a file that is 47:59:59.999999 days old is rounded down to 1 day which is not strictly greater than 1.

Again, both GNU and FreeBSD find support suffixes as extension, but again with different semantics.

For those though, there is already a different (non-standard) API that we can use with a more intuitive API: ! -newermt '1 day ago' as a more intuitive equivalent to -mtime +0 (same as FreeBSD's -mtime +1d or +24h/+1440m/+86400s).

The -newermt was originally from BSD.

I also asked my question here:

https://stackoverflow.com/questions/71200433/linux-find-by-size-bug