It is inherent in people to assume that somehow offering people more will make you appealling to more people. It's just not true. When you try to be all things to all people, you end up being very little to very few. People need to know what you're about. Keep your menu focused.
Large menus lack focus. When you try and offer EVERYTHING your customers like, you aren't giving them more choices and more reasons to come back, you are confusing them. They don't know what your specialties are, what you supposedly do well, what they should order, and how to describe you to their friends. If your message is focused and easy to convey, more of your customers will convey your message.
Large menus take longer to order from. The more choices you have on your menu, the longer it takes each table to peruse that menu, and the longer it takes for them to order. For every minute they are NOT ordering, you are NOT making money for the seat they are occupying. Take this statement to heart if you want to be successful in the restaurant business: You will only ever be as successful as your peak period of service. 80% of revenue, and 100% of profit is made during peak periods. Anything that limits your ability to serve customers and collect money during your peak periods is limiting your potential for profit.
Large menus require more inventory items. The more items on your menu, the more ingredients you need to buy to make those items, and the more items you'll have on your shelf. Every item on your shelf represents a possibility for loss. It can be stolen, it can be mishandled, mis-prepped or stored incorrectly and spoiled. The less inventory items you have, the less waste you'll have. The less waste you have, the more profit you'll have.
Large menus require more equipment and personnel to produce. The more items you have on your menu, the less opportunity your staff has to cook multiple orders at once. Less multiple orders means more burners, grill space, fryer grease, and hands are required to produce the same number of dishes. All these additional tools cost you money.
Large menus mean longer ticket times. When you have too many different dishes cooking at once, and less multiple orders in the same pans, it means more time to produce whatever is being ordered. Beyond the fact that Americans are no longer willing to wait 45 minutes to have their dinner prepared for them, you should be thinking about how long ticket times limit your ability to process people through your dining room. The longer it takes to serve each table, the less tables you can turn during peak periods.
That's a lot of things to worry about, and that's only the tip of the ice berg. There are many other managerial concerns. This is why I'm telling you that your great ideas for a menu, and incredible talent for cooking will only get you 1/3 of the way to operating a successful restaurant.
How many people I'm feeding each day/shift/hour
What items they're buying, and how many of each
What gross profit those items are contributing
What those items should have cost me to sell
What my actual cost of selling those items is
What my labor is compared to my budget
How many labor dollars I spend per sales dollar
How many labor hours I spend per sales dollar
What I purchase each day, and how to categorize each purchase for analysis
What my sales are compared to what they should have been
What my profit and loss is for EACH WEEK