Is there a grammatical (or semantic) reason that explains why the opposite of 便利（な） is 不便（な）rather than 不便利（な）。 I'm asking because I noticed a similar thing possibly occurring with 満足 and 不満 and was wondering if there was some underlying information that could help me better understand Japanese grammar.
Many on-yomi kanji compounds were made by combining two kanji with similar meanings.
- 満足: 満 ≒ 足 ≒ suffice; satisfy
- 便利: 便 ≒ 利 ≒ convenience; advantage
- 明瞭: 明 ≒ 瞭 ≒ clear; visible
This is because the on-reading of each kanji is very short and people needed two kanji to disambiguate. For example, マン by itself is too ambiguous because it can mean ten thousand (万), chronic (慢), etc.
But if you add 不, two kanji is often long enough and you may no longer have to say two similar kanji. There is no such word as [*]不万 (non-ten-thousand?), [*]不慢 (non-chronic?), [*]不勉 (non-study?) or [*]不弁 (non-valve?).
For some compounds, you cannot omit kanji at all (eg 不連続, 不確実). Or omitting one kanji may change the meaning a bit (eg 不明 = unknown vs. 不明瞭 = obscure). Basically you'll have to memorize each word individually.