Over a sample size of 300 tournaments, my ITM% is 22% but my ROI% is -25%. I feel like this gap is telling me something about my play. For example, maybe it means I'm min-cashing often but not making enough deep runs and winning bigger amounts at final tables.

Is the sample size too small? Or is this a sign of overly-cautious play, i.e. playing too passively to make the money instead of taking risks to try and build a big stack.

If the fields are very large, stats like these will be common over 300 tournaments. If its small fields, then possibly you are to focused on surviving and would benefit from adopting more of a "go strong or go home" mentality.

The sample size is definitely too small.

Over a larger sample though, if you had a high ITM and negative ROI, that would definitely indicate that you were playing too tight/cautious and needed to sacrifice some min-cashes for the opportunity to finish in the top few spots.