There is size="20" set on <input> type text, search, tel, url, email, and password … by default, which is approximately of 100px width, although it can vary in a different browser and operating system.
On the parent, you have min-width:15px; set, that does not take any effects, because the value is much smaller than 100px. If you change it to width:15px; or max-width:15px; you will then see the differences.
Alternatively you can give the size a very small value such as size="1".