The title "The main different" pretty much sums up what you should do next.

Sorry for picking on the petty grammatical error. I'm sure you meant "difference".

Workload, of course, will be different for master's and phd. You can't expect STPM-level workload for UPSR-level classes, right?

Candidates proceeding from master's to PhD usually are those that decide to stay in school. Typically they continue their master's level work and develop it into a PhD-worthy work. Most times that would mean wider and deeper background work (literature search, critics of state-of-the-art methods, etc). Then the methods will be more thorough and evolve into something more complete. Usually full-time master's level research work can be completed in one year, while a phd-level research work can range between 2 to 7 years. It is not unusual for part-time phd work to take 10 years.

This is of course very general information. The best way to get information about the program you're interested in is to get in touch with the students currently enrolled in that program.