Originally Posted by Moredread
It seems both have a problem with bigger fonts (for example for a headline)
I don't think you should be having a problem with large font-sizes.
Have you tried playing around with the "Font size key" option in Calibre [Convert] - [Look & Feel]?
If you haven't used it before you might need to read up about it in the online User Manual
Also, for your info, it's a while since I tested using RTF as a source file in Calibre, but I seem to remember that Centre- and Right-align didn't convert correctly and neither did line-breaks. Converting to HTML also solved these problems.