There are two ways of getting control over the amount of work. One way is to break things down in equal sizes. Big things are broken down and small things are put together. If they are really small just do them instead of putting time on putting them together. The other way to get control of the amount of work is to estimate. Estimating does not change the variation of size but it helps you get control over variation on the difference between estimated volume and real volume.
It's important to remember that there is no science in estimating and there is no way to get it perfect so don't look for perfection and don't connect estimation with incentives or commitment. It will only lead to problems in the form of vagueness, bad quality due to shortcuts taken to meet a deadline, and unreasonable high estimates with low engagement energy from the team.
There are though some ways to improve your estimates:
The people that will do the work should do the estimations...