printing sizeof() values

I'm trying to print out sizeof() values for some of my structures. They are coming out incorrect. Here is a small table in which I've done this on different platforms:

Linux : sizeof(TChannel) = 1460
Windows: sizeof(TChannel) = 1460
8051 : sizeof(TChannel) = 1361

Are there byte-alignment issues perhaps? I have both Linux and Windows defaulting to a 1-byte boundary for byte alignment in structures. Does the 8051 default to something different?

Here's my code for the 8051:
Debugf( "sizeof(TChannel) = %u\r\n", sizeof( TChannel ) );

I've tried %u, %d, %lu, %bu, %X but can't get the right value. Here's my Debugf() function in case that might be messing things up:

void Debugf(BYTE* format, ...)
{
#ifdef DEBUG
xdata BYTE buf[64];
va_list arglist;
va_start (arglist,format);
vsprintf(buf,format,arglist);
va_end (arglist);
SendSerialData( buf, strlen( buf ) );
#endif
}

I don't think the problem is in my SendSerialData() function as that seems to work well.

Any ideas?