Small web application doesn’t require architecture design. It means server script, html, client script can be put on single file for one web page. On the other hand, large business and scope with much more user, change requirement occurs so many times requires good & reliable web architecture design. It is just not simple any more to put everything on single file since the web may update and grow often.
When business requirement become more complex then related web application usually consume more programmers.
Those programmers can work per module i.e. manufacturing, finance, sales, shipping modules etc. One programmer / team work on manufacturing and other programmers / team work on other modules. It depends on software designer, sometimes work module could be mix.