The break cue shaft is the same size as my playing cue. As far as unintentional spin I am pretty good at parking the cue ball right between the side pockets so I don't think it would be spin. The tips are different, I use a Tiger Everest on my player and I have one of those clear Bulletproofs on my break cue.We all love dirty 8b on beater Valley barboxes!
There are several variables besides velocity that could have been contributing to you getting better breaks. Does your break cue have a wider shaft/tip than your playing cue? (Larger sweet spot). Or a harder tip, or flatter tip? (less inadvertent spin). Any/all of these might be allowing you to get less spin and be more accurate - giving you just a tiny bit more of a square, solid pop into the head ball.
I’m not into break cues, but if i did have one, I think I’d prefer a fatter, conical taper (vs the 12.5 mm pro taper I prefer on my playing cue). I’ve noticed when power slamming on barboxes with dirty/sticky balls and using house cues, I seem to get better results when the cue has fatter, stiffer taper.
This might be what some folks term “more efficient energy transfer”. Or maybe it was a few extra beers and your shoulder was looser - of course that may also be technically considered more efficient energy transfer