There is no accounting for tastes! 
I for one, when I first unwrapped my FP4, was happy about its solid feeling weight. It doesn’t feel like a small piece of cheap plastic, made to fall apart if you give it a hard stare. Note I don’t have to worry about its weight deforming my expensive designer clothes’ pockets, and I’m a burly guy able to lift those 225 g with a single arm. 
As for the size, the FP4 is a little longer but just as wide as my previous (very old) Samsung Note, and for me that was a condition to buy it: I’m well past my half century and need a big screen to be able to read anything without glasses. Smaller phones are useless to me (yes, I don’t want to have to pull out my reading glasses each time the phone rings, thank you very much).
TL;DR: I for one am happy with both the weight and the size, and wouldn’t had minded even heavier and bigger either. “Jedem Tierchen sein Plaisirchen”… 
If Fairphone should focus on an issue, it definitely isn’t that one, there are many way more pressing issues to be tackled. IMHO.