Sometimes there is a mismatch between the webpage width and the screen width. Can that be adjusted by tweaking something in the (Page) Source (Code) ?

Not sure what you mean. If it does not happen with every page, then it sounds like an issue with the page. But then again, are you sure it is just not maximized? Note too that many website programmers intentionally set page width so viewers using standard (not widescreen) monitors, or even cell phones, can view it properly. Perhaps a screen shot to illustrate what you mean will clear things up for us.