Why does the condom seem loose at the end of my penis shaft, right before the head?

[David]

There is a difference between extra head room and reservoir tip.

The reservoir tip is effectively meant to collect semen.

What you are referring to however, is probably the extra head room. Indeed, some condoms are designed with extra headroom to provide extra comfort, it tends to follow the regular shape of a penis that is bigger at the end. They are commonly called form-fitting condoms. Now some condoms have extra headroom shapes and others don't. If you don't like it, you can choose a condom brand with a regular shape (no extra headroom). These condoms are commonly called straight condoms (available with or without reservoir tip).

Usually you can see the shape of the condoms on the box, otherwise you can look online. Most online stores give specifications about the shape of the condom.

example of different shapes : https://www.lacapoterie.net/eng/condoms.php