Q:And why are Sabers [sic] normal sized?!
A: Longswords are multi-purpose craft, operating both in space and land operations. Their armament is substantial, requiring a larger craft to carry such a heavy payload in atmosphere (hence the nickname 'longsword', a larger weapon than a 'sabre'). The YSS-1000 (usually simply referred to as the Sabre) on the other hand is a space-only superiority starfighter, requiring much less structural space for weaponry.
Q: Well, why is the Longsword a "fighter", not a "heavy gunship"?
A: seeing as a gunship would be a hornet or a falcon(gunship implying it has the ability to stay stationary like a helicopter), and a longsword would be more along the lines of a fighter/bomber, i really cant say