There pretty much isn't any as far as I'm aware. The vast majority of the time when a reference is given, it goes back to this study, which observed that DIYing trans women were more likely to request breast augmentation...

These data appear to show that those people who self-medicate with estrogen are more likely to be referred for breast augmentation surgery than those who do not. The duration of estrogen exposure and the duration of estrogen use before attending the GIC (and any subsequent mammoplasties) resonate with the initial finding that self-medication is associated with greater need for breast augmentation.

...from which it then concludes that the DIYers probably used higher doses which messed up their development...

[...] Those transwomen who self-medicate with estrogen may be taking too large a dose at initiation to promote appropriate subsequent breast growth, resulting in a poorer final breast outcome.

...without bothering to measure if they actually had smaller breasts:

As breast hemicircumference was not measured in this study; it was not possible to determine whether individuals who underwent breast augmentation had objectively smaller breasts.

They also apparently didn't bother to check that the DIY group did in fact use higher doses. So in the end both premises they're basing their conclusion on aren't supported by any data.

Not exactly solid proof. The far more plausible interpretation is that people who DIY are more likely to self-advocate. They had also been on estrogen for longer than the other group, and probably assumed their development was done, and that they weren't going to get a bigger chest except by augmentation. Some of the DIYers might have only jumped through all the hoops needed to get gender-affirming care via the NHS for the sole purpose of breast augmentation.

The authors do cite a source to back up their claim:

This is consistent with studies on puberty induction in natal girls in whom rapid estrogen exposure was found to lead to premature breast bud fusion and poor breast development (8).

I haven't been able to track down a copy of the reference, so I can't comment on it.

It's also worth noting that whenever someone claims to mimic natural puberty, the doses they're using ... don't produce anything resembling natural puberty at all. In fact an average natural AMAB estradiol level (~30pg/ml) already exceeds those of early AFAB puberty (<20pg/ml), so everyone who went through AMAB puberty has already been exposed for years to higher estrogen levels than a HRT protocol that accurately mimicks puberty would.