Because the tiny one sucks and there's no other way to resize it.
Add this to your bookmarks:
javascript:(function(){var newcss=".k:not(.dg){width:1280px;}.aR{min-height:500px;}";if("\v"=="v"){document.createStyleSheet().cssText=newcss}else{var tag=document.createElement("style");tag.type="text/css";document.getElementsByTagName("head")[0].appendChild(tag);tag[(typeof document.body.style.WebkitAppearance=="string")?"innerText":"innerHTML"]=newcss}})();
Click it when the compose window is up.