Hi jojo
I wouldn't say you have to be worried hun. I am at the same stage as you and while my siblings tell me I'm huge and my sister's b/f in particular tells me I am having twins for definite (despite all the evidence to the contrary eg one baby in the scan), my colleagues, all mothers, tell me I'm quite tidy. Your bump looks bigger from where you are looking anyway! I find that when I lie down, a little bump is visible but not as much as when I am standing. The thing is, everyone is different. Wouldn't you be worried if you weren't growing too? A bump isn't all baby anyway, as I'm sure you can feel. My bump comes all the way up to under my breasts now, I can even rest my teacup on it...but my baby is only moving around about half of that, i think the rest is just organs being pushed up. I've never been measured for fundal height or anything but I think it's just about a couple of inches above my navel at the mo. And once you get to 5 months the baby is doing a lot of growing too.
I'm sure you're just tidy and people at work haven't a clue. I've noticed since I got pregnant that your size is suddenly a matter for public comment, though nobody would dare to comment on it if you weren't pregnant. Another example of people not considering your feelings.
I will try to post a photo of my bump soon, but I'm on my own today and can't get the camera still enough!
*HUGS*
Sue