The Quantity and amount sizes of 15 are hardcoded in qty_cells() and amount_cells() functions in includes/ui/ui_input.inc and are visible in the rendered page source code attached.
The outer table is 80% of the browser width and the inner table 90% width of that table. The field's column cell widths dynamically expand but the actual input fields remain the same size.
Also refer:
https://www.tutorialrepublic.com/faq/how-to-expand-select-box-to-its-default-width-on-focus-using-css.php