As a hiring manager, I would probably ask you to supply the months, so I could get a better understanding of your work history. I'd want to know and be able to ask you about any extended periods between employment. My rules of thumb tend to lean toward:
- One job ending in the same or subsequent month the next job begins means you likely left of your own volition.
- Two to five months difference may signify the job loss was not your choice, but you had the initiative and/or the talent to be able to find another job relatively quickly.
- Six months or longer will definitely cause me to ask you to explain the gap.
If there is education or skill development that covers all or part of the time period, I could readily assume you were focusing on your studies, which may or may not leave you sufficient time and focus for a quality job search.
All that being said, you may come across a hiring manager that isn't looking as closely as I would to these types of details. Whether or not you include or exclude the months is completely up to you. Historically, I've never excluded them personally, and I don't recall ever hiring someone that didn't include the information. With that in mind, I would never reject an applicant who didn't supply the information, and I know I've never summarily rejected an applicant who didn't provide the months if their work experience met the qualifications I was looking for. I guess what I'm trying to say is if you really feel the need to trim down content, be more concise with your job duties.