I often wonder why some pregnancies end up with ginormous bellies and others with small (and let's admit, disappointing) ones!
Sometimes the same woman have different sizes, think Linda_online.
Apart from multiples, what makes a huge belly huge ? I read somewhere that second pregnancies are supposed to be bigger, or that older women tend to have bigger bellies, but I'm quite skeptic.
Is it food? Smoking/alcohol hindering baby's growth ? Is it hormones related?

What do you guys think?

Frame, genetics, and amniotic fluid levels. Those are the main three and something that's out of the woman's control.

(August 18, 2023, 6:19 am)Conatus I read somewhere that second pregnancies are supposed to be bigger, or that older women tend to have bigger bellies, but I'm quite skeptic.

Neither of these is necessarily true, but it is true that women can get bigger earlier with subsequent pregnancies or when they're older. Pregnancy stretches out the abdominal muscles quite a bit. The muscles of a first-time mom in her 20s are going to generally resist belly growth more than a second or third-time mom, or a mom in her late 30s or early 40s.

I knew a woman who had four pregnancies while I knew her, from her late 20s and through her late 30s. She always reached about the same size by the end, but she always hit that mark a little earlier, and with her 4th pregnancy she looked full term for the last 3 months.

(August 18, 2023, 7:36 am)Proctoersone Frame, genetics, and amniotic fluid levels. Those are the main three and something that's out of the woman's control.

Also important: size of baby and position/orientation of baby.

Genetics and diet.

A woman’s height can also affect how her uterus grows. For example, In a short woman, the uterus will grow outward more because it has less room to grow upward. This will produce a belly that protrudes noticeably. Also it will look a lot bigger because of her small frame.

A tall woman’s uterus will have more room to grow upward which will produce a less protruding belly that looks small.

Also, you have to consider the body of the father. Huge guy knocking up a petite woman is going to produce a large baby in a small frame.

(August 18, 2023, 4:16 pm)zklone Also, you have to consider the body of the father. Huge guy knocking up a petite woman is going to produce a large baby in a small frame.

I don't think that has to do with anything. I am a big guy (6'4) but I was tiny at birth and I weighed less than 6 pounds at birth, and caught up later. I don't think a man can influence what happens in a woman's body

(August 18, 2023, 8:59 am)doubleintegral Also important: size of baby and position/orientation of baby.

I dont think the latter would make that much of a difference. Yes it'll change the shape of the belly, but not much as the size.

(August 18, 2023, 4:16 pm)zklone Also, you have to consider the body of the father. Huge guy knocking up a petite woman is going to produce a large baby in a small frame.

The father's body isn't a factor when the woman with a small frame is going to have a large belly regardless.

(August 18, 2023, 4:39 pm)garadon I don't think a man can influence what happens in a woman's body

Not completely true, in fact, a man's sperm can definitely influence what happens inside a woman's body if she is ovulating...

I can only really echo what others have said over the size of the woman and how many pregnancies she has had previously. Those were the first factors which come to my own mind. Naturally, every pregnancy is different. From my own broad observations, I tend to find women with their first pregnancies generally, although not always, have smaller bellies. One common trait I find is that due to tighter stomach muscles (unless she's overweight to start off with), there's more of a reluctance for her belly to grow and they can still look generally small even at 25ish weeks. 

Some women can be quite lucky and have a few pregnancies where they can 'snap back' into decent shape quite quickly. I work with someone who during her own two pregnancies looked more or less the same size by the end of both (from what I could tell anyway - there could of course have been minor differences) and within a year/18 months after the pregnancies were over, you'd be hard pushed to tell she was even a mother. 

On a slightly different subject, although still relevant to this topic, is that I often wonder what goes through their minds as their body changes - or when they discover that they're pregnant. For instance, I know that some genuinely look forward to developing a bump. I don't often see or hear from those who are apprehensive or fearful of certain changes - although I'm sure there must be at least private thoughts.