Thats the point of sieges in terms of real life - you lawyas need at least 3x troops number to succeed in frontal attack.

Try to do next steps to win a siege:
1) Obtain best weapons and armor you can (jedi weapons from trade fed outsposts and rebel armor - best choise)
2) be sure you have at least 6-8 companions with good outfit too
3) your troops limit must be at least 60 and filled with high-tier soldiers, otherwise its instafail even if you'll be very lucky
4) your and companions skills should be properly developed - power strike, shields, ironflesh, should be 4-6 minimum 9at least power strike)
5) Optional: bring few bacta shots and thermal detonators with you

Best troops for assault - militia and sec guards due to their shileds and pistols, imperial officers and royal guards as a steady backbone, rebel vanguards, wookies and of cause jedis, bring as many jedis including initiates and padawans as you can. Never use pilots on sieges because they posess weak weapons and armor, thus being very weak w\o theit swoopbikes which will be 100% shot down inm seconds after siege begin.