The maximum compression ratio of gzip

_{Update 2020-02-06: As mentioned in the comments, I have been unable to reproduce the original result with gzip. Working on the assumption that I accidentally used a different compression format in that original quick test I've repeated with gzip and updated the figures below accordingly. This new result agrees with the theoretical maximum compression stated in other answers/comments.}

It very much depends on the data being compressed. A quick test with a 1Gb file full of zeros using a standard version of gzip (with default options or specifying -9) gives a compressed size of ~1018Kb, so your 10Kb file could potentially expand into ~10Mbytes.

If the data has low redundancy to start with, for instance, the archive contains images files in a format that is compressed natively (gif, jpg, png, ...), then gzip may add not further compression at all. For binary files like program executables you might see up to 2:1 compression, for plain text, HTML or other markups 3:1 or 4:1 or more is not unlikely. You might see 10:1 in some cases but the ~1030:1 seen with a file filled with a single symbol is something you are not going to see outside similarly artificial circumstances.

You can check how much data would result from unpacking a gzip file, without actually writing its uncompressed content to disk, with gunzip -c file.gz | wc --bytes - this will uncompress the file but not store the results, instead passing them to wc which will count the number of bytes as they pass then discard them. If compressed content is a tar file containing many many small files you might find that noticeably more disk space is required to unpack the full archive, but in most circumstances, the count returned from piping gunzip output through wc is going to be as accurate as you need.