You can customize how to display the number. For example, define the following function that formats numbers as
(defun jp-rel-format (offset)
"Another formatting function"
(format "%03d: " (abs offset)))
Then customize the variable
relative-line-numbers-format to use this new function instead of the default
You may try other format strings such as "%3d" to avoid leading zeros.
However, I just installed
relative-line-numbers-mode and I don't experience the jump with numbers beyond 100. Are you using variable pitch font?