What are the major technical differences between Google Chrome and Chromium that makes Chromium running on my Ubuntu smoothly while Google Chrome is expanding and killing both my CPU and RAM ?
Chrome is the brand product by Google - the base browser AND specific modifications. It generally does a lot of things that are widely considered to be violating the privacy of its users or the fundamental principles of free & open software - hence is not the default choice for Ubuntu users.
Chromium is the software base of all the non-Google-related browser features which is shipped with fewer phone-home features and not many modifications beyond the base browser.
If you wish to find out why they are performing differently, you want to one-by-one disable all Google-specific features in Chrome. You may or may not be able to disable the delinquent, which may be among plugins, codecs, error reporting or search features.