How to say "You have this much work to do because you decided to do that much" more elegantly?

There are a few common idioms or sayings that fit the situation rather well, though they do not mean exactly “having so much [work] because you chose it yourself”.

There’s “You made your bed (– now lie in it/you are going to have to lie in it)”, which means that the person you’re talking to has created their own mess, so now it’s their own responsibility to fix it.

Similarly metaphorical is “You reap what you sow”, which compares the bad situation to a field—if you sow bad seeds, you’ll reap bad grain; likewise, if you ‘sow’ a bad situation for yourself, you’ll have to ‘reap’ the consequences, no better.

More direct and pragmatic, “You asked for it!” is used to indicate that someone should not be complaining about their current situation, since it is only what they themselves asked for in the first place.

Even more direct is “You brought this on yourself”, which does not mince words and simply says that the person addressed caused their own misery themselves.

Note: All of these could be considered somewhat rude—their intention is to be blunt. If you wish to be softer and more polite about it, I don’t think there’s a fixed expression available that fits. Instead, I would just phrase it in a natural, but polite, way; something like, “I’m sorry to hear you’re so caught up in work—but you mustn’t forget that you did ask for quite a heavy workload, so you really only got what you asked for”.

An idiom that is a bit less rude would be Be careful what you wish for—you might get it (or it may come true, or lest it come true; there are several variants), which quite properly warns someone not to want or ask for something without first thinking through what actually getting it will entail.

You could say "You bit off more than you can chew.", which means he tried to do more than he was able to.

Logically speaking, "You bit off more than you can chew" is the correct answer. But conversationally, you might hurt the addressee's feelings. Maybe he/she was just looking for your sympathy. Or maybe he/she wanted to politely decline your invitation. Context, context, context.

Maybe "you can't win them all" would be (if somewhat imprecise) a more sympathetic version.