The use of the "pea sized" blob is extremely dangerous. It can be dangerous because it can vary in size 5-fold or more. It can be dangerous because you might underdose a medication that is critical to your animal. Or it might be dangerous because you might overdose a medication, causing side effects or even death. Even though most deworming medication has a fairly wide safety margin, I have seen severe side effects in about 25% of chickens when they were accidentally given 20 mg/lb (instead of 20 mg/kg) of Panacur for 5 days. Of those affected, about 40% of those died!
But it's not just the blob-size variability. There's a separate issue that might be even more important. These medications are made for a 1000 pound animal. Because they are made to be used in relatively large volumes, they aren't always mixed thoroughly enough to accurately predict the amount of medicine in a tiny volume.
Think of this like cooking. When I was a young girl I was "helping" my aunt bake a coffee cake. The recipe called for 1/4 teaspoon of vanilla to be mixed into a large amount of cake mix. I asked how such a small amount of vanilla could distribute into every drop of batter. My aunt said that it had to be mixed well enough to be sure that there was some vanilla in every bite, but that it didn't need to be mixed so thoroughly that there would be vanilla in every crumb. When we took a bite, we would taste the vanilla within that bite, even if there were a few crumbs that didn't have it thoroughly mixed in. That's why you just need to spend a few minutes mixing cake mix with a fork, not an hour mixing it with a blender on puree speed.
So applying that to medicated paste intended for large animals, the paste is mixed adequately such that there is a known amount of medicine in a horse-sized volume, comparable to a bite sized piece of coffee cake in the above example. However, it may not be mixed well enough to have a know amount of medicine in a bird-sized volume, comparable to the tiny crumb of coffee cake in the above example. Some pasty "crumbs" may have a high amount of medicine, some may have a low amount of medicine, and some may have no medication. When I lived in SC and GA years ago, several people I knew with large hunting dog packs wanted to economize on heartworm prevention. Instead of buying the appropriate heartworm prevention made for dogs, they did the math and gave the ivermectin horse paste to their dogs. Every month most of their dogs got enough medication, but some months a dog wouldn't get enough or any medication with their paste, and would get the parasite. The cost of treating just one dog for heartworms is more that it would cost to prevent the disease in 30 dogs for a year, so people quickly found out that it wasn't worth the risk. Theoretically, yes, it should work. But sometimes the practical details outrank theory.
Liquid medication made for large animals, either solution or suspension that is very thoroughly shaken, is a different story. Those medications should be adequately mixed to measure down to a chicken-appropriate volume.
