That is a great question. Kiva goes to a lot of effort to make sure that all Terra Bites are approximately the same size (and therefore dose). However, you are correct, that despite their efforts, not all of them are the same size, and therefore your assumption is also correct. They all contain approximately 5mg per bite, but not exactly. The cannabis is infused into the chocolate (not the espresso bean) and so more chocolate does mean a larger dose.
Fortunately it's also pretty easy to see which are larger and which are smaller, and so you can compensate yourself if you are careful in your selection.