How to make walls thicker using meshmixer or meshlab
Only today, I learned of a solution for this sort of objective, but it uses Fusion 360 rather than Meshmixer  or Meshlab. As your question does not include that program, I'll toss the Meshmixer method.

This image is of the model prior to modification:



When you load your STL file into MM, use Edit, Generate Face Groups. This will cause the surfaces to change color. Click Accept.

With face groups created:



If you can be assured of all one surface, use Select, then double click on the interior. This should turn the entire interior red. If you discover unselected surfaces, simply click on those surfaces until all is completed. If you select a surface in error, use Shift-Click to clear that one surface.

Once selected, the select menu gives you a new edit menu. 
Use Edit, Offset for yet another menu. As you make changes in the menu settings, you'll see the results on the model. Ideally, you won't have an overly complex model with too many facets/triangles, as it can really bog a machine down.

This particular model has a nearly uniform interior. Double clicking on the inside surface caused the full cylinder (not the bottom) to be selected (turning red).

Low accuracy offset, with surfaces still selected:



For smoothest results, keep the accuracy high. Any protruberance in the interior will give very strange results.

High accuracy results, surfaces selected:



Experiment with the settings, aim for the best result and click accept. As long as you don't export the model over your original, all experimentation is a learning experience and not a destructive one.