Why is OS X Maverics upgrade size the same as a fresh install?
You seem to have two questions here…


  Why is OS X Mavericks upgrade size the same as a fresh install?


There's only one installer. Unlike Windows, there's a single OS X installer that is able to install on and/or upgrade any machine capable of running Mavericks, from any prior version of OS X all the way back to 10.6.


  Is there a way I can download the upgrade of smaller size or do I have to get new installation in any case, and then how should I have a resumable download?


No, the OS X Mavericks install is the single installer available — there's no smaller copy.

The Mac App Store does support resumable download, so don't worry about your internet connection dropping. It will pause/resume the download automatically.