As someone who has worked at all levels of restaurants, I can absolutely assert that there is no comparison between the guy carrying your $10 sandwich and the guy carrying your $50 Porterhouse. I worked at Applebee's years ago, and I could serve 8 tables at once with little trouble.
Where I work now, a table of 2 usually ends up spending about $180. If I get more than 3 tables, I consider myself uncomfortably busy. There is just too much to be done. There are several 'courses', and the entire table must be cleared/crumbed/rectified after each course. If a guest gets up to use the restroom, we lead them there in person, and then race back to carefully fold their napkin while they are gone. We are almost always a few feet away from the table. We hang their coats for them, we push in their chairs for them, we bring their valet tickets out, and hold an umbrella over their head on the way to the car if it is raining. We go through the requisite elaborate wine-bottle opening ritual so that they feel good about their $300 wine purchase.
At crApplebees, I could show up half asleep and do my job just fine.
As a side note, we never add gratuity to a table, no matter the party size, but this policy has really burned some servers in the past. We are near several large hotels and often serve guests from overseas. If you are given a party of 10 from the U.K. or Scandinavia, all the other servers immediately pity you. Basically, it COSTS the server money to wait on these folks. We have to tip our bussers/bartenders/backwaiters based on sales, a typical and fair practice in many restaurants.
So...let's say a 10-top comes in, spends about $1000, and leaves $50 for a tip. This is VERY common from foreign guests. Now, that server owes about $15 to the bartender, $18 to his busser, and $25 to the backwaiter for that table. He received $50 in tips, but his tipout to others is $58. He has worked 2 hours on this table and lost $8 for it.
In addition, the IRS ASSUMES that he has brought home at LEAST $80 based on his sales, and taxes him on that amount.
The sad thing is that, often, folks from these countries are the most courteous of any of our guests, and will tell you what wonderful service they received as they leave the restaurant, which makes it harder to be mad at them.
As a server, I watch other servers very closely, and I must say that almost all of our disney servers did an excellent job. There was one, at Liberty Tree Tavern, who was obviously very busy. The problem is that she kept TELLING us that she was so busy. I expect that at the Prime Time Cafe, where it's part of the act, but otherwise, it's very unprofessional. That is nitpicking though, and it didn't affect her gratuity.
I'll also mention that, at every level of service, lousy servers don't last long. It only takes a few complaints for someone to be shown the door, and Disney is known for their customer service, so I would imagine they are even more strict about such things.
If you ever have bad service, it is important that you tell the manager. If you have had gratuity added, tell the manager while you are still there. If not, then by all means leave a lousy tip, as you should, and notify the manager via email later on. Email is great because you can write it all down and not lose your cool over the phone.
I use to pay whatever the original bill would have been before the 20% discount, saving me from doing any math. 
