I want to study in Canada. Are the chances of getting visa for 1-year Diploma course less than getting visa for 2-year Diploma?

Not really, your education qualification, relevancy of course, future plans will be more important. If you have completed 10+2, you should aim for a Bachelors and not another Diploma. In this case of course, visa officers will be curious to know what exactly you plan to do with your future.