As per the description on the website we send them small on purpose. It actually means the cup you received was fresher than others. If you give them just 3-4 days at room temp or keep them a little warm, they will be huge in no time! The main cost is the food to grow them, so when you buy large you are actually paying more for the food not so much the worms.
Medium hornworms are shipped in medium cups, with enough food to grow them to medium size and maintain them for a period of time. To avoid them being too big for a lot of owners to feed to their reptiles and having to send out replacements, we send them this way to allow owners to grow them to the size they need. If your hornworms arrive smaller than expected, simply leave them at room temperature for up to 3 or 4 days, or a bit warmer for less time and they will have grown exponentially before you know it! This way you get your hornworms for a lot longer of their life cycle and get your money's worth.
One great way to actually maintain the size of your hornworms is to grow them to the size you need, then keep them at 50 - 55 degrees Fahrenheit, that way their growth will slow down and they will eat a lot less of the provided food but they will stay alive, and this way you can keep them for longer before they get too big for your reptile! You can buy more hornworm food if you are looking to maintain them for longer.