A jpeg output transparent with WMS

[stephSITSIG]

Hello world,
I have a few questions about mapserver. 
We are testing the mapserver 5.0 with a geotools 2.3 client as a user
interface.

We use Mapserver with Raster SHOM Data.
Each layers ( 5 ) are tiled (gdaltindex) and overviewed (gdaladdo -r nearest
2 4 8 16) with tif raster data (SHOM). These SHOM display some area of
French-Polynesia. These Layers are scaled with parameters minscale and
maxscale.
I have try the ouput drivers GDAL, GD and AGG ( Gif, png (24/32 bits), jpeg
and jpeg2000) for output WMS requests with screen size of 600*600 pixels.
Is there any FORMATOPTION for tuning the size of these output or other
tricks ? 
Because :
 with GIF format , i get an image size of 400 KBytes ( IMAGEMODE PC256),
 with JP2K format => 500 KBytes,
 with JPEG format => 900 KBytes,
 with Png format => 1 MBtyes.

Another thing, the transparency :
All layers in the mapfile are scales (min,max), but when my BBOX go out of
all scales i have different results depending of the OUPUTFORMAT. For
rendering the output, i use Geotools 2.3, i have in the user client, three
layers :
The topmost layer are Vector data compute by a GEOSERVER,
The middle layer is my SHOM data compute by the mapserver,
and the base Layer are Vector Data representing the island's coastal edges
of  French-Polynesia computed also by a GEOSERVER.

So when BBOX is out of scale ,
i only see the topmost layer with the JPEG Driver, it seems that the result
of the mapserver is not Transparent => is there any possibility to render
the base layer not hided by the SHOM Data layer with a AGG/JPEG driver ?
With the other format (GIF, png) i can see the topmost layer and the islands
=> because they can do Transparency.
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