Ouch. I've heard this a lot from parents of multiple children (mine has been an only for nearly 9 years) and it too often sounds to me like, "Yeah, when I just had one, I was a conscientious parent like you, but now I can't be bothered." Makes me feel sorry for their younger kids! I guess it has a lot to do with context, though: I've most often heard this kind of thing not when I'm over-worrying some minor issue but when I'm defending myself for doing something (like limiting junk food, using the car seat correctly, or putting bug repellent on my kid) that the other parent is arguing is unnecessarily fussy because he/she stopped doing it after the first child.
I can really relate to the comment about the coin. You're expecting your second after 8 years with an only? I had a second after 10 years! I definitely can see using the phrase or sentiment to a new mom in a totally nice way -- because I can look back on some stuff that I stressed that just wasn't necessary - and I was a relatively low-stress parent. No need to feel sorry for the younger one in our family. The benefit of a parent with better perspective far makes up for the occasional swallowed dime. ;-)
I can see why you were put off by the sentiment if it was used to belittle something you felt was important but that's not my experience of trying to convey this or having someone try to express it to me.