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”.