question on new psql datatype

I am developing a new "image" datatype in postgres which contains a binary 
field for storing image data and some other fields for additional information 
about the image, like size, resolution, etc. I was hoping that the clients can 
saving their time by directly retrieving these information from the fields 
without retrieving the whole image, that is, the time for reading these fields 
should be constant with respect to the image size. However, the result shows 
that the time for direct retrieving of information from the fields of an image 
datatype is still increasing with the size of the image. My explanation is 
that in order to read the fields of a image datatype, the whole struct would 
still need to be loaded into memory, which includes the binary field 
containing the actual image data. Can anyone tell me is this true for a user-
defined datatype (using C struct)? Thanks a lot.

Yu