The experience of filling out a form in a mobile device environment is substantially different than filling out a form on a laptop or desktop. There is no mouse, or even (often) a Tab key to navigate between form fields. Big fingers and tiny options in pop-up menus make for a bad match.
A form that might work fine in a full-sized browser may well be terribly uninviting and inconvenient in a mobile device. Take Apple's store locator (shown in the following screenshot) for example, it is poorly designed for mobile users, requiring mobile users to invoke their frustrating keyboards to enter a zip code into a tiny text field:
Part of the solution to that challenge is that mobile devices often convert form fields to more accessible elements. For example, options in a select menu might display much larger when tapped, making it easier to make a selection. The same Apple form I just criticized has a better option for choosing a product—a drop-down menu that is easier to...