How can I pause one big installation package operation and install another small package then resume paused package again. I'm using ubuntu software center to installing the big package. Is this possible?
You mean still downloading, Yes but not exactly pausing.
Click cancel (X), Install other package, then back to previous one.
APT keeps partial downloaded packages in
This is not possible. The best you can do is to cancel the installation with the red X button (on the Progress tab) and then start it again later.
Partially downloaded files are stored, so if you decide to start the install again later it should resume downloading where it left off.