Here in my area, my group of friends uses the term "you're doin too much" and variations as an idiom to say exactly what you're asking. Literally what you asked provides the exact definition of the term my friends and I use constantly that I've wanted so long but haven't been able to describe what we mean to my parents, as we're teens in college (started using the term years ago though). The term is frequently used to generalize a person in the now, so a lot of times it's used in present tense regardless of when the act that defined someone as "doing too much" happened, but only in the context of already having stated what this act was. The idiom in general is specifically used as a way to express that, in your opinion, it's not worth doing something so trivial. As a result it's commonly used to lightly make fun of someone by putting down an act they did, even if the act was justifiable from his/her perspective (i.e. "you walked 2 miles all the way to the store and back just for some milk? You're doing the max."). From that person's perspective, the act my have been justifiable for them to do it, even if they agree the act is dumb, because they may have been forced to do it (their parents may have made them get milk and they didn't have a car so they had to walk) so it's not generally used specifically as a way to put someone down. However, even if the term is used to put down an act committed, even in the same context as the example, it doesn't have to make fun of the person being spoken to if the person speaking acknowledges that what he/she was doing was justified from their point of view (i.e. "your parents made you walk all the way to the store, 2 miles, in the rain for some milk? They're doing wayyyyy too much", targetting the parents, or "your parents made you? That's doing the maxxxx", targeting the act itself and used in an agreeing, sympathetic context).
As time progressed the term "doing too much" has evolved to also work with variations with synonyms to provide variety in what we say and display individual difference from others so we don't all seem the same by saying the same thing all the time (because we use it so much from it fitting so many situations so well). For instance:
"Doin the most" - not used as much by us nowadays, but has been seen in pop culture so it may be used by others in other areas.
"Doing way too much" - nowadays this is usually what we say instead of "doin too much" if we're going to use it because it better states the reason of saying it, unless it ends the sentence (i.e. "...blablabla, that's doin too much." Works better in that situation than inserting "way")
"Doin a LITTLE bit too much" - used with extra emphasis sarcastically to mean the same thing as "way too much"
"Doin a little bit TOO much" - similar to the last one but the extra emphasis on "too" instead of "little" changes the meaning a bit. Usually starts with the common starter for this idiom "That might be..."
"Doin the max" - by far used the most nowadays, prob the best variation.