I haven't had quite what you describe CR

But I did start on a bike whose frame is slightly too big... and it felt great (my racey red Scorpio RA 300 which is now my training bike)

My partner kept insisting it was too long for me, and got me a purty silver and blue Giant TCR Aero (my race bike)... I had barely ridden it 50 metres when I realised how perfect the fit was...

As my miles slowly build, I am changing the seat height (higher) and the bars (lower) so i can be more aero on it.

I also find as my belly reduces (rather slowly) that I fit differently on the bike too...

That probably doesn't help you specifically with what you desrcibe, but maybe it does demonstrate that how we (well, me anyways) need to grow with our bikes and they with us...???

I'm so unsure if this is helpful I am very close to not posting... but I will anyways...