Vol.73, Chapter 771
Vol.74, Chapter 778
Not sure if there are any more, but these are the two scenes I think of immediately.
(After some more research, it's likely the original phrase is "ママも痩せてたけどわりと胸はあったから" in 778. "Mama was also thin, but she had relatively [big] chest."
I got this quote from two separate Yahoo questions. It is likely the posters quoted directly out of the manga, thereby confirming the use of past tense. Source here and here.)
Anyways, most of the points from those articles are already considered in Chekhov's theories (his first post in this thread,) and if not, they are too wishy-washy to be worth your time and you're better off reading Chekhov's posts. I only posted those links as a sort of pseudo-confirmation for the usage of past tense, before I get proper confirmation from someone who has access to the raws.
With all that's said, I find it very strange Sera would be using past tense here. (For those not as familiar in Japanese, the phrase here sounds as strange as it is in English.) The only times I think of using past tense in this situation is either when her mother is dead, or she's grown fat, and/or she's lost her "relatively" big breasts.