She's a grade-school teacher. She probably, being proper, took Mokou's measurements with the rest of her class yearly.

or she did exactly this last year and erased the history

if she can grow her chest while be immortal she will end up with super huge monster breasts by the end of time...

OK! TIME TO BUILD A TIME MACHINE!

Meh, it probably just changes with her diet.

Eirin: Kaguya I'm going to need to measure you to make sure you Costume is properly fitted.

Kaguya: Very well.

Eirin: *Take measurements* It seem that your Breasts have shrunk 2cm from last year.

Kaguya: WHAT!?!

Mokoppai - delicious oppai.

1Kero2Kero said:
Eirin: Kaguya I'm going to need to measure you to make sure you Costume is properly fitted.

Kaguya: Very well.

Eirin: *Take measurements* It seem that your Breasts have shrunk 2cm from last year.

Kaguya: WHAT!?!

Conservation of boobage. It's not just a good idea - it's the law!