Justifying a smaller control group in survey study I am planning a set of studies that are interlinked. The first study collects information from 400 participants (exposure group), then some psychometrics are performed to assess the questions. In a second study, we plan to collect a control group sample (unaffected/typically developing) to make comparisons with the exposure group using ancova (group difference, plus covariates). A reviewer has suggested that our control group could be reduced to be much smaller.
Is this justified as the sample size required to detect our minimum clinically meaningful effect size is only around 150 in total. The 400 exposure individuals is on the basis to be able to conduct the psychometrics and adequately capture the variability in the exposure sample.
should I specify a smaller control group and use some sort of matching (n-to-1 matching, using all exposures but a smaller proportion of controls), and allow for the unbalanced groups in the sample size calculation? I'm not completely sure on the justification for this.

I think you've answered your own question.
If your goal is to independently assess the psychometric properties within the control group, collect enough data to do that. If your goal is only to compare the exposure and control groups, and that requires less data, just collect enough for that. If you have two goals, work to whichever requires the most data. Make it clear in your paper what your goal is (or goals are) to justify the sample size you do go with.
Another possibility worth considering here it is evaluating the psychometrics for the control group, but rather than analysing this data in isolation you could use, e.g., multi-group factor analysis to test whether the psychometrics differ between groups. This would require less data than two separate psychometric analyses.
