Can you control the file size output from gdal_merge?

[jamesdudden]

Hi,
I need to merge some raster files and have tried the gdal_merge utility and all worked well but the end file size was much bigger than the combined filesize of the indivdual tiffs i.e. 3 X 15MB tiffs = 1 X 150MB file.

Is this right or can you control the file output size?

Any help or suggestions are welcome

Regards
JD