Pharmacies use standard vial sizes for many of the medications they compound, and fill the vial based on the specific prescription. This system requires less tooling and machinery changes between compounding batches.
A 1mL volume could be in a 5mL vial (1/5th full), as could a 2.5mL (1/2 full) or any other volume.
If following your prescribed dosing schedule, you will not run out of medication early.