Is there really ever a time that is appropriate to tell someone, "gosh you are really big"? No, no there is not. And some people think that when one is pregnant it is free reign to comment on appearance. I am pretty sure no one ever wants to be told they look really big. And if a person is big, or bigger than they normally are, they know it. Trust me, you informing them of their additional size is not enlightening them to something totally new.
I think the most dangerous time to comment on someones size is when they are pregnant. There are many more hormones racing around a very tired, overwhelmed, and exhausted body. Especially, when one has gone to much trouble to shower, and appear presentable all whilst wrangling two other small children who are hell bent on making sure their mother never looks like she has seen a hair brush or makeup.
Let me give you some advise, only say things like: You look wonderful. You are so tiny! You look amazing. OR novel idea, say nothing at all - haven't you people ever heard "if you don't have something nice to say, then don't say anything at all". PLEASE. Yes, I am oversensitive, I am pregnant with my THIRD child. I do on some level understand that these comments are not intended to hurt my feelings, and the person offering them is only trying to make a keen observation. But lets talk about the weather instead.
Okay, I am done with my rant. For now.