After using my 7Plus for a month, using my GF's 7 does cause a little eye strain. But so does going from my iMac after a few hours to my 11" air or iPad Air 2. For the record, the text size on both devices are the smallest.
The largest causes of eye strain are out of focus text (either b/c of the clarity of one's device or impairment of one's vision) and prolonged usage. Size of screen has zero to do with it.