The size must be quoted when including the unit specifier, eg:
alter database HCP on HCP_data_001.dat = '10G'
'alter database' does *NOT* modify the attributes of the disk so if it's currently defined as 104GB in size then it will remain defined as 104GB in size.
If this fails, please post back with the entire output from sp_helpdevice (preferably as a *txt attachment if possible).