I'd speculate on how this may have happened, but templates like that are forbidden!

Eirin looks like a grown woman in this one, or at least 16. But in the next one she looks 8 again, is this a mistake?

Goldilox said:

Eirin looks like a grown woman in this one, or at least 16. But in the next one she looks 8 again, is this a mistake?

I think it's just a trick of perspective. Eirin's much closer to the camera in that panel, while Reisen's on the other side of the table.