Is there any reason to amass a mafia larger than 501?

Yes, absolutely! I can think of several reasons to continue adding people to your mafia. Your family is more than just an entourage of bloodthirsty goons ready to fight for and protect you.

There are several advantages to having as large a family as possible, even once you've filled the maximum fight mafia size of 501. Many of my arguments for growing your mafia as soon as possible also apply to continuing to grow your mafia ad infinitum, so we'll start with those and add on all the other benefits of having a huge family:

• More people in your family helps you purchase Mafia Mike's properties which are 1) cheap, 2) earn decent income and 3) cannot be robbed.
• You will also be able to purchase plenty of Limited Edition Loot (usually just $10000.) You never know when something amazing will be offered. They rarely stick around more than a few days so you don't want to be left scrambling for mafia members to buy what you like.
• You earn 3 experience every time you fight as one of your family's capo -- the larger your mafia, the more often you'll see you've fought as a capo & get passive experience!
• The more mafia members, the more often you'll be able to give help on jobs when they call for backup. No only will this get you money and occasionally collectible items without having to spend any energy, you'll also be able to get more experience per day than just what your energy allows you to earn by yourself. At higher levels where it takes progressively longer to level up, this is a huge boon.
• More likely to have maxed out candidates for your Top Mafia.
• More likely to have family members use you in their Top Mafias, giving you an added bonus.
• Better chance of someone gifting you items from your wishlist.
• No one in your mafia can fight or rob you -- might not be a significant % of Mafia Wars players, but every little bit helps!
• Pride, vanity, ego and/or generally feeling badass! :)