A fed mid lane does at most 2x the damage of an unfed mid.
Meanwhile a fed adc does 9x the damage of an unfed adc. This is the prime reason why botlane has too much impact. There is way too much difference between adcs in a match if there are even a few kills. Let's not also forget that if a bot lane is fed, the support also has more gold, and a fed support has way more utility from items than an unfed support.