Dorian:
I'm glad someone's pointed this out. I've been noticing it for years. There's a HUGE overlap between girls who gain and girls who have cut.
Ultimately - both are technically self harm... and they're both being done for the same reasons.
Psychologically many people cut as a temporary relief from anxiety; specifically to drown out feelings of uselessness and ineffectuality. They're cutting to deafen out the silent voice that is constantly making them feel like they have no self agency.
In such deafening silence they'll voluntarily take the dopamine hit from inflicting physical damage - because it momentarily makes them feel like they are in control of their own life and surroundings.
Gaining is essentially the same thing. "I might be ugly/stupid/lonely and don't have the talent/confidence/willpower to improve my own life.... but at least I can gain weight! At least I can gain self esteem and self actualisation from succeeding at that...even if its technically self-harm too!"
Thats basically the long and the short of it.
I'm concerned by how certain you feel on this, as if it wouldn't vary from person to person. This is a long and somewhat drug-fueled ramble but my own personal theory as to why it's so prevalent is a combination of this and the reasoning behind why transgender individuals have such a high suicide rate.
Obviously being a feedee/gainer is not the same as being transgender, but in the case of gainers it's not difficult to imagine that living "in the wrong body" and being told by society that what you find attractive/what you want to be is undesirable would lead to low self-esteem and self-harm. This is purely anecdotal but I've noticed that a lot of gainers also have had eating disorders in the past, and while this can be a "chicken or the egg" situation with their desire to gain, many of us have had this fetish since early childhood, before we understood the concept of an eating disorder. I think knowing your desires are taboo leads to internalizing subconsciously the belief that if you get thin/attractive/muscular enough, you'll be able to hide from your desire to be fat.
Going on with feederism causing low self-esteem and not low self-esteem causing feederism, the whole media outlook on feederism seems to try to paint it as your exact opinion, where mentally unstable women are being coerced by men to become seven hundred pounds. Knowing that your "sexuality," so to speak, is viewed that way by society could definitely lead to cutting if that's how you cope with negative self-image.
My other thought is that maybe fetishes are correlated with mental illness in some way, in that people who are born with brain chemistry predisposed to developing mental illnesses are more likely to be "broken" sexually. This means that gaining and depression are correlated but in some individuals, neither inherently causes the other; it's just that people who gain also happen to be depressed or bipolar or schizophrenic. Maybe this means feederism is a mental illness, but there's not much research on any of this (for the sole reason that it's taboo), though again, anecdotally, I've noticed an overlap between people with autism and people with fetishes in general, and if that link could be verified in a scientific manner, it'd lend more credence to this.
My take doesn't apply to everyone, but I don't think yours does either. As with most of mental health and identity, it's more of a spectrum.