An homogenous mesh will yield a better analysis. Individual tetrahedron that have a large relative volume can impact on the accuracy of the analysis.
Tetra with extremely large volume are detected with the Mesh Repair Wizard : 
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Description
Internal nodes may not be evenly distributed in unmatched thick regions. This will result in some tetras being much larger than others. As calculation results of adjacent tetra is taken into account when analyzing a model, large volume tetra can affect the solution resolution in chunky areas.
Example of tetra with large volume
In order to find large elements, we define volume ratio as the volume of a particular tetra vs. the volume of its biggest neighbor. The default value for this ratio is 20. Any tetra with a volume ratio over this threshold is said to have a large volume.
Impact on the analysis
The accuracy of results in regions of sudden thickness change may be affected by tetra with large volumes.
Recommended action
Auto fix addresses this problem by reducing the size of the tetra to below the threshold value where possible. Unless there are unexpected results in thick, chunky areas (possibly including change of flow direction), it is not compulsory to repair this problem.