I’ve searched and have yet to find any mention of this, so excuse me if this is already public knowledge.
It appears as though IE7 applies the min-width value as width on input elements with type attribute values of button, reset, and submit.
Example: https://steve.ganz.name/examples/Forms/min-width-input.html
According to the CSS 2.1 specification, min-width can be applied to all elements but non-replaced inline elements, table rows, and row groups
. As far as I can tell, the input element is a replaced element so min-width should apply.