I find quite the opposite: The more things I do, and the more productive I am, the more rapidly things get added to my todo list. It's when I'm less productive that my todo lists stay short.
It's an interesting phenomenon that (at least my) todo lists are effective sticks for the carrot in that the size of the todo list scales with productive output.
this is also what I find.
Another interesting bit is that I seem to actually finish between 10% and 33% of the tasks I plan to have finished by a certain date. This percentage completed doesn't seem to vary much between the times when I get a whole lot done, and the times when I get very little done.
I mean, I think my 'level of irrational optimism' remains fairly constant. I believe myself to be between 3x and 10x better than I actually am, and while within that range my error band is pretty random, it rarely steps outside of that range, even though I have weeks where I easily accomplish 100x what I would accomplish in my worst working week.
(I'm only counting time when I'm actually attempting to be productive. It's not fair to compare my "productivity" while working to "productivity" during vacation unless you want to introduce some very difficult to quantify variables.)
Work expands so as to fill the time available for its completion.