The reason why you're publishing is because you think you have a new way of doing things that is better than the old way of doing things. Simply publishing a derivation of the new way isn't sufficient to convince people of that fact. You need to convince people that your new way is better.
So yes, your derivation is a simple one to two paragraphs, but in addition to that you should spend several paragraphs explaining why your new way is better. Why was the old way used? Why weren't the deficiencies not considered a problem until now? (Or if they were, how were they addressed with the old way, and why is your way better?) What are some of the objections that the people used to doing it the old way will raise? Why might they not want to switch to your new way? What do you say to those potential objections? Are there possible use cases might your new way not be appropriate for, and if so, how do you choose between the old way and the new way?
You'll probably also want to spend several paragraphs in an introduction, explaining how the field got to the old way, and outlining the deficiencies. You probably also want to include and example of using your new way, comparing it to the old way, illustrating why the new way is better.
Basically, you think your paper is too short because you're planning on just presenting the derivation and assuming its merits are obvious to others. They won't be. In all likelihood you'd be chewed up in review - not because your paper is too short, but because you won't have convinced the reviewers that your new method is worthwhile. Fleshing out the paper with necessary background and discussion to presents a coherent argument for your new method can easily take you into the 1-2 page range which is the typical size of "brief communications" (depending on field, journal, etc.)
Don't pad your article just to increase its size, but don't omit things which will bolster the arguments in favor of your new method.
Edit: In response to comments, here's an attempt at clarification: An academic paper is not just about presenting results. It's also about presenting a story and an argument. How does this result change the field? How should it change how the readers think about the topic?
I'm guessing that the original questioner thinks their paper is only one to two paragraphs because they were anticipating presenting just the short proof/derivation of the new figure of merit. My point was that the proof itself is insufficient for a decent paper. In addition to the proof you also need to convince the reader that the new figure of merit is better than the old one - and to do so for people who may be unaware that there was anything wrong with the existing way in the first place. Doing this properly can easily extend the paper from 1-2 paragraphs into a size which is more typical for a standard journal article. Write a proper paper, and it's no longer "too small to be publishable".
