In my point of view, it's preferably chosen by either restrictions or considerations of many related issue(s) simultaneously. Like being adaptive for mobility, design concept, development capabilities, the length of the form, etc.
As I suppose, most of the websites have fixed input length because of the symmetrical concerns leading to their
design aspects. It's a common use case while there are comparably less (2 to 5 mostly) input box displayed together like a sign up form or similar forms.
download bmml source – Wireframes created with Balsamiq Mockups
Hence, if it's not the case, you probably have a bigger form where your users need to fill a detailed personal information. In this case, you probably don't see any example displaying all same sizes of inputs.
download bmml source
Even you haven't visualized your exact output here, I think it mostly depends on the preference,
- of symmetrical concerns (design simplicity)
- restrictions on the space saved to display the form
- even because of the incapable developers designed that page
You may feel free to display the outlook of your own choice yet considering the limitations of your circumstances.