Many if not most of those you see in stores have Doritis in their lineage and will be smaller growers. HOWEVER, this by no means is to say that they will stay truly mini. Most will not. Small and mini are two very different things. That is what you must accept when buying generic 'puppy mill' type plants. If you REALLY want mini plants, your only recourse is to buy them from a reputable grower or a hobbyist you know well and trust. Upon contacting the grower you need to make it very clear what your criteria are. True mini's amongst the phals are few and VERYfar between.
Phal. equestris is a small grower, but definitely not a mini -- though a happy plant will start blooming at a very young age (and hence at a very small size).
Phal lobbii and Phal parishii are both true mini's as is their hybrid, Phal Lovely Kid.
Phal Zuma Pixie has remained of mini stature for me.