I don't use MudBox that much, but I know if you use inflate style brushes you will get this sort of effect. Also it can depend on your stroke. Also, don't be too quick to start adding masses and large forms to the model, try cutting into the model.... almost like drawing in the forms.... I'm no pro, but seems to be a general theme that i have noticed with some sculptors. Cut in the details and then add mass where needed.... I personally am enjoying the more 'violent' method of hacking in the forms. Racking away at the masses, and building up masses the same way you would with real clay... but obviously you use other tools to help.... anyway, if you could post a screen grab of your model/brushes, and what setting your re using.

Try cutting in the forms, and when you go to add mass..... don't do it all in one stroke, add it in several strokes.... try and imply the direction that the shape/mass/muscle should be running, and try and imply tension, and relaxation as well.

Also, try to remember that everything is built up in layers.... once all the basic forms are there... then all the rest (specific muscles/ fat / flesh / etc) are all laid on top in layers.

P.S: Yeah, the mouse could be contributing to the problem. As you have no control on how hard the sculpture will be edited (except for the manual pressure control)... with a tablet you can give your strokes a gradient of pressure along the surface, allowing for a more fluent look, and feel. With a mouse however, you get a more even form, and feel, and this usually make the forms look 'blobby'. As the parts that attach to rest of the sculpture (in between muscles, and other forms, where it should look flat), usually have the same amount of mass as the parts that should have the most mass.... and in order to try and counter this effect, you will use several strokes with the mouse, which can create lots of 'blobs' everywhere.

Sculpting with a mouse isn't impossible... but It is a lot harder.