What's up with this composer?

Not one like Handel or Bach (sorry, Mom). I mean the Blogger composer, the box in which I write these blogs; the box which allows me to place links and add italics for emphasis, etc... For some reason, every time I use our Mac laptop to write, it screws up all the formatting. Even if I jump on the PC to try and fix it, it doesn't seem to interpret the text in the same way. So if any of my posts have really large or small or a blend of font sizes, it is the computer's fault. It is in no way a lack of understanding on my part.