I don't know, if this is the right place for such a discussion maybe some moderators will move it to a better matching forum.

This issue also isn't an issue to be discussed only on Rendereosity, it should be discussed everywhere where 3D models are sold.

In the upper upper most cases female models come with breast either as large as the base model with 'an average size' or larger. But if you look around the real existing women, e.g. your working mates or girls laying around on a beach, sitting with you in the subway or … or … or …, you will for sure notice, that approx. half of them have breast which are smaller. And I think, that the most of those girls are proud of their small boobies.

To reflect the reality approx. half of the female adult models should come also with smaller breast. But this is not the fact.

Is there any specific reason for this?

The only exception I found until now is the V4 based Francesca where the creator had been inspired by a real former photo model which was really proud to show their small breast.

How other customers think about this? Would you also buy female figures coming by default with smaller or even not perfectly shaped breast?

Body form is up to the artist making the figure. You can adjust that with body morphs (morphs++ for V4) or with aftermarket morphs. Biala has a variety of breast morphs for G3 & G8. I suggest using the Francesca body as a starting point for V4 figures. You might also try a teen body. They're often more petite. Bodies are malleable. You may have to make adjustments to get what you want.

In my opinion, I could care less simply due to the fact that if I'm trying to achieve a certain look or body/head proportion, I have plenty of morphs to make the necessary adjustments, and there are plenty of breast morphs available. The only thing I noticed with female characters in the same series is many of them have the same general breast size/shape. My guess is the artists try to stay somewhere within a median between large and small because its easier to adjust up or down from there as opposed to having to adjust from the one or other extreme. I think I recall seeing some of her photo sets years ago, if its the same woman. Some of the themes of her photo-shoots I found to be rather disturbing and would probably get the photographer thrown in jail in some countries.

As noted above, there are a lot of add-on morph sets to remedy this problem. As to why Artists usually go for larger breasts... well, the fact is that they're trying to sell a product and bigger breasts tend to sell better. That hasn't always been true... in the 1920's the "hot" look was almost no breasts at all and the flappers of the period bound their girls down if they hadn't been fortunate enough to be natural a-cups, for example... and we're actually on a downward trend again compared to the extreme breasts of popular actress of the late 50s/early 60s.

My personal fallback is usually to dial in just a small bit of Zev0's Growing Up morphs (sold at DAZ), which not only takes the character's breast size down a bit, but also reduces width of the hips brings their height to something a little less statuesque.