Try
data = fread(fd, '*int16');
When you use 'int16' for the type to read, MATLAB reads as int16 and then converts to double (four times the memory). When you use '*int16', MATLAB keeps the type as int16. E.g.,
% Create a small sample file
fid = fopen('junk.bin','wb');
fwrite(fid,int16(4),'int16');
fclose(fid);
% read it with 'int16'
fid = fopen('junk.bin','rb');
x = fread(fid,'int16')
class(x) % double, not what you wanted
% read it with '*int16'
frewind(fid);
x = fread(fid,'*int16')
class(x) % int16, this is what you wanted
fclose(fid);
