That much loss in two weeks is significant. imo Before troubleshooting vials vs. pens, I’d focus on understanding what went wrong: was it the nausea and GI effects, or was the appetite suppression itself not right? Mounjaro dosing was designed for T2D, not BED or restriction history.
Sometimes a lower dose works. Sometimes the tool just isn’t a fit. On your actual question: smaller vials seem discontinued. Your prescriber can adjust the schedule though - you don’t have to escalate every two wks if effects are rough.