Honestly, all vial sizes have their fans, but I think it really comes down to dosing and storage habits.
General guidance is to keep a mixed vial for around a month, mainly for sterility and potency reasons.
At the top clinical amount, a larger vial basically lasts about a month, but at half that amount it stretches closer to two months.
Smaller vials, though, run out much quicker. Same math applies, at a moderate weekly amount, the tiniest vials might only cover a single dose, while a mid-size one lasts roughly three weeks.
See where this is heading? A moderate vial size tends to land right in that ideal zone for low-to-medium amounts, long enough to be worthwhile, short enough not to sit around too long.
Naturally this hinges on the actual weekly amount someone runs. Someone running a higher weekly dose does better with the bigger vial size. And plenty of people stretch vials out three months regardless, so an even larger size works fine for them too.
Lots of variables at play here. The biggest factors are individual response and time.
Everyone reacts differently to the dual-action compound, some notice appetite suppression at very small amounts while others need to increase quickly. Depending on that and how long someone's been running it, they might eventually reach that ceiling amount, or they might not.