Here's how TMDSAS suggests to handle this issue. You could list it as 2 job entries (fulltime MA in the summers, with your fulltime hours per week and start/end dates) and (Part time MA, with your other shorter hours per week)
4. Employment
☐ List all jobs (paid work experience) held since graduating from high school to the present, including military service. Paid healthcare activities may be listed here in addition to the healthcare activities section.
› Indicate the employer, job title, when the job was held, start date, end date, city, hours worked per week, country, state, and a brief description of the job (300 characters). Click on [Submit] to save the record.
› If you held a job every summer, enter each time period as a separate record.
› Additionally, if you held this job for consecutive terms, select “Other” for the question “When did you hold this job?” You will be able to provide more details if you wish.
› Additional records can be added by clicking the [Add an Employment Activity] link.