SAR terrain correction and mosaicking problems

I am processing SAR images of Arctic sea ice from different sensors to an user defined polar stereographic coordinate system: PROJECTION[“Polar_Stereographic”],
PARAMETER[“central_meridian”, 55.0],
PARAMETER[“latitude_of_origin”, 90.0],
PARAMETER[“scale_factor”, 0.969846],
PARAMETER[“false_easting”, 0.0],
PARAMETER[“false_northing”, 0.0],
Terrain correction with Ellipsoid-Correction-RD.
Works fine, but the resulting images have large blank borders, e.g. here a TerraSAR-X example:

What is the reason for this? When I use UTM41N projection instead the geocoded image is a tight fit around the original image.
Second problem is with SAR mosaicking, it also results in very large blank areas, e.g. here two consecutive (same orbit) TerraSAR-X UTM41N coded images:

Why is this happening?
These blank spaces can be removed with Subset tool, but you have to do it manually.

Hi, could you solve hat problem, as I have the same issue ?!