Yes i agree with you that Designed website for mobile not having too much size but also considered that it does not having to much content also because its not worth for user. User generally can not read  more information about website they only read required information.

Of course because the resolution of any mobile is not bigger than the PC . So if we make website for mobile than consider that you only put information which only require not extra.

Now in days the best hot news is that Content is now more important than Design. In my opinion only. Because content will give you a good ranking in seo also. Its increase your website prestige also.

 if you want to convert it in JOOMla than
We want to set up a tree structure as you see in the following image:

You can see three folders, CSS which contains template.css and other style sheets including ie_hack.css files, Images which contain all your template images and js which contains some java script files if you decide to add some. Then you have your typical index.php, template_thumbnail.png and templateDetaisl.xml. All these folders and  files are very important and all must be included in your templateDetails.xml file.

A Joomla 1.5 Template is setup using 'Positions', a position in Joomla is an area like your Main Content, Left Navigation, Right Navigation, Header, Footer, User1, User2 etc. Of course there is more to modules and scripts that can be added but we are just going to focus on the basic setup and conversion from plain html template to Joomla 1.5 template.

Remember all Joomla 1.5 templates contain an xml file like the one previously mentioned. If you don't include the xml file no file will be uploaded to Joomla 1.5 backend and it will display an error as you upload your template. Remember any file you add to the zip folder must be included in the templateDetails.xml file with the correct path i.e. <filename>images/back_container.gif</filename> contains a file name back_container.gif inside images folder.
Now that you have a fairly good idea on what a position is in Joomla 1.5, let me show you how you would create the positions in a Joomla 1.5 template from a html template.

Yes i agree with you but if you want more inoformation about this than you can also visit this page netmagazine.com/tag/responsive-web-design?ct=feature