Something that also makes me believe in Chreon is the fact that Leon bulked up between RE Death Island and RE9.
So throughout the years, Leon's weight and build varies drastically.
He has a kinda slim-fit build in RE2, which makes sense for a 21 year old who's been working out between studying.
In RE4, he's bulked up a bit, showing the intense military training he was put through for presumably the last 6 years (with saving Ashley being his first mission under the gov) but he's not crazily bulked up, showing he's working out but not getting excess calories.
In RE Infinite Darkness, roughly 2 years after RE4. Leon's slimmed down, his arms are definitely smaller than both iterations of RE4, showing after that mission in Spain, he stopped the military training.
In RE6, he's kinda slimmer than RE4, but he's still got a fair bit of muscle, showing that he's started working again out but maybe (again) not eating as much as when he got his RE4 build.
In RE Vendetta, he's lost all of that again, he's skinny and barely has any muscle and he's obviously having a hard time with his mental health.
In the Japanese novelisation of Vendetta, it's stated that Chris helps Leon with his mental health by talking to him about what happened and drawing on their shared experiences with losing teams.
In RE Death Island (especially the manga) Leon has bulked up again to levels similar to RE4 OG and overall seems to be doing better compared to the last time we saw him, he's bigger and has more muscle, he's smiling more, and he's making his quips again.
After Death Island, next time we see Leon I believe is RE9, where he is older, broader and even more muscular than he was at 27. And i think it's because of Chris' help.
My theory is that after Vendetta, when Chris reaches out to Leon to help, they start getting closer as friends. They maybe meet up for some meals in diners where they can, as well as working out together occasionally, encouraging Leon to bulk up between Vendetta and Death Island. At some point, they start dating, and at some point after Death Island, they move in together and get married. Living with Chris prompted Leon to start eating what Chris does, (meals presumably higher in protein and calories) and to start working out with Chris more often (maybe they've got a home gym).
This explains why, instead of continuing to fluctuate, Leon's physical health only increases from Vendetta onwards, leading to a physical prime at 49. He's being looked after by his husband who's like 6'2" and built like a brick house.
In RE8, Chris is fed up with being on missions, not only because of the BSAA's betrayal, but because he's forced away from his home with Leon for longer periods.
They've both got a home and husband to return to.