Yeah, Celestia didn't get much dialogue here. Though, on the inside, she's clearly just as kinky as Derpy, if not more so.

Well that took something of a left turn.

I did enjoy it, make no mistake; Celestia's new look is basically made of my fetishes.

But we went from a pair of breasts which possess the theoretical potential to become "enough boob" (see my previous comment), to a four-titted futa with a side order of giantess and some stomach-bulging insertion for dessert.

Like I said, basically made of my fetishes (don't judge me, you're here too), but it was kind of sudden and might turn away some people who are only here for 'Moar boob! Moar milk!'

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Well, the amount of dialogue is somewhat intended if only to show Derpy that she was right to think that getting that big that quickly is more overwhelming than her long term project. I'm just struggling to be satisfied with the tone.

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Yeah, it's a bit of a shift. I'll continue to focus on Derpy in future stories, this one served as an excuse to establish a greater degree of control over her boobs and to give her a larger permanent boost. Although I'm not ready to be pinned down on whether it happened because someone very powerful had her milk or if it's some kind of psychological thing because Derpy wanted to impress her so much.

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New life awaits you on the off-world colonies! Get on that space program, Celestia.

Now I want to see Luna try some of Derpy’s milk

Definitely hoping we get more princesses involved in the coming chapters! I'm interested to see what the changes might be for them, and how large Derps is gonna get as a result of it. If they do show up, my guesses would be:

Luna: a figure more like Celestia's compared to her own more lean build. Breasts / butt / hips and some height to round it off.
Cadence: Motherly figure, fertility (and if Shiny's on the other nipple, have him want things that would make him a better father and a better lover).
Twiggles: .....? Not sure what her motivation would be.

Really excited to see where this goes. I'd love if there were 'latent' effects to her milk that cause those 'improved' aspects of ponies to continue growing / improving over time (or with additional doses of Derp milk). Can't wait for the next installment!