This is almost always a mismatch between how you have your blueprint set up and how you have your artwork set up, and is generally easy to fix.
Let's say you used a simple yellow rectangle as your blueprint artwork, and positioned it on products like this:
The fill of the artwork stretches to the edges of the canvas, and to get it to fit "right" on the shirt you scaled it to roughly 75% of the space.
Now imagine you try to use this blueprint to exhibit artwork that you have designed for Merch. Merch requires a 4500x5400 image that covers the entire canvas, so you put transparent space between the "actual art" and the border; in effect, you've pre-scaled the artwork to 75%. When combined with the blueprint instruction to scale to 75%, you're scaling the image twice:
Thus the print ends up being way too small.
There are two rules you should keep in mind when setting up a blueprint:
The artwork you use for your blueprint should resemble, as much as possible, the artwork in your design library.
If your artwork is designed for Merch, ignore rule #1. Upload a 4500x5400 image and always scale it to very edges of the print area on every product. This is easiest to do with a tall rectangle that fills the canvas like the yellow one above.
What if I've already messed this up?
If you've already exhibited a bunch of designs and they are appearing too small, don't worry! This is easy to fix.
The easiest solution is to just take your existing blueprint and adjust the scaling properly on each product - typically so that the edges of the image are at the edges of the print area. Then re-sync exhibits from the Blueprint page in OrbitKit:
Alternatively, you could point your existing blueprint at a new design (which has been set up correctly). Look for the "Select" button on the Blueprint page. Then resync.
Caution: Never use an exhibit that was created by OrbitKit as a blueprint!