If you have overlapping features, the dissolve tool will merge those overlapping areas and make the overall sum of the polygon areas smaller.
This doesn't mean all the overlaps have disappeared. Only the overlaps between identical values must have been cleared. Please see example below to understand the process :
In this example, you have
- 3 polygons : 1 polygon A, 2 polygon B, all identicial
- 100m² each, so overall area is 300m²
- all overlapping each other
When i dissovle on the Name field, I get
- 2 polygons : A name polygon, B dissolved polygon
- A polygon and B polygon still overlap
- overall area is 100+177 = 277m²
I guess that is what happend to your data and that is what explains why the area has decreased.
The conclusion is also that your data must not be topologically correct. Please check your data to avoid overlaps to avoid this kind of results (@GISGe & Joseph have hinted this answer, and I agreee with them).
The usual arcgis overlay tools can do a lot of things between layers but not inside a layer, that's why they don't detect any problem.
You need to solve the overlapping problem and make sure there is no multipart feature (it can be painful dealing with those).
A efficient way to solve your problem is to separate your values in as many layers as unique values after dissolving, then unionize the layers all together. You'll be able to have a single flat geometry (and have the area), while not loosing the overlap information.
in my example : you need to get the dissolved A+B layer, then split in A layer and B layer. Then you unionize A layer and B layer.
you'll be able to know what is the A area, what is the (A AND B) area, and what is B area.
It can be time-consuming if you have a lot of unique values.
BONUS (if you don't care about the attributes)
If all your data must be dissolved together with no attribute distinction, just leave the optional 'Dissolve fields' parameter empty, and you'll have a big dissolved polygon with no overlaps
(Thanks @GISGe for the tips)