I am reading an unpublished paper that employs diff-in-diff to firm data containing a total of 2598 observations. Of these observations, 2474 are in the treatment group and 124 in the control group.
I read here that the control group should not contain less than 20% of the total sample. But clearly, in this published paper the control group contains less than 5% of the sample. Also, the absolute size of the control group also looks small to me.
I would like to understand if the results of the diff-in-diff in this paper can be reliable given that the control group contains less than 5% of the sample.