I post these only when I realize that I have actually heard one, or used it myself, in real life, and when it isn't one that's on every "list of idioms."

"You've bitten off more than you can chew" means "you are planning to do too much." I used this one myself. A person said she planned to visit national parks in South Dakota, Wyoming, Utah, and Colorado, all within one week, <em>in February.</em> She's young and energetic but I think it is probably too much to do in one week, especially if there is bad weather. (That's 2000 km. of driving, some on mountain roads, at a time of year when there can be snowstorms.)

"You've bitten off more than you can chew" is used when you've made the plans yourself.

"Too much on your plate" or "a lot on your plate" can be used when you have a list of tasks you've been given, such as work assignments. For example, "We'd better not give him that task, he already has a lot on his plate."

Both of these are food metaphors. The "food" represents tasks you must complete, the "plate" is the list of tasks, and eating them represents doing the tasks.

There is also an idiom that's used literally with food. When I was a kid, sometimes I put too big a serving on my plate, and didn't eat it all. My mom would say "Your eyes were bigger than your stomach."