The Overflowing Bra

This clocks in at just under 8,000 words.   It's a nice length, but the first few pages are painful to read.   Once it gets going, it improves.
Characters -- glaringly terrible in the first few pages and while they do improve, it only gets to a point where you have some nice cardboard cutouts for a fun situation.  The boring backstory at the start would be better replaced with some character defining moments.
BE description:  There's some unsettling word choices in there to offset some decent transformation moments.  "Her baby fat shifted, onto her hips."  RIP my boner.  There's some nice attention to clothing damage which brings up the score quite a bit.
Technical Writing -- The dialogue in this is laughably bad, and the narration states the obvious so much.  There are also some poorly chosen words at times:  "Her feet waved excitedly, like a child waiting for sweets."  Feet don't really wave.   They kick.
Overall, I really can't grasp any coherent direction to this, no mystery or logic to the action.  Dumping a bunch of random back-story on the reader doesn't really make a great story-- at least the 'action' is decent in this more often than not.   Might be worth chucking out for people with witch fetishes.
  

I'm sure there some interesting story written deep within this, but I can't get to it. The beginning is confusingly written, poorly set out, and generally a bit discouraging.
events are going in an odd order, parts that should be drawn out are rushed and we are put straight into the middle of it with little reason to care.
My advice is this, if you mean your story to purely be entertaining, then don't put in so many plot points, you confuse the reader, misleading them as to what has relevance. If your story is meant to be more of a story then just a quick fap session, then bring out your characters a bit more. Let us really get to see your characters through how they interact with others first,(perhaps we could get some scenes of the apprentice interacting with the witch before things get serious) before introducing the meat of it. I hate to use the phrase: "show, don't tell" because its reading to begin with, but its really relevant here, don't tell us the whole background through a long overview, inform us through character interaction.
Maybe this is just my inner literary critic showing though, I just get excited when I see someone with good ideas throw them to waste