Is it proper to ask an editor for clarification
There is certainly no problem contacting an editor to enquire about a manuscript and review responses. It is difficult to assess the issue in your particular case but I do agree that adding new data can be a significant endeavour indeed. Shorting the paper may be a relevant point in any case but I do not see, in general how, shortening a paper would improve any lack of data although it may be obvious in the individual case.

So, if you feel the advise you have received is difficult to follow or is unclear (and that seems to be the primary point), then asking for a clarification is in place. You can be constructive and also suggest some plan of action for the parts you see more clearly and then focus on the ambiguous parts in your request to the editor.