Program Memory

I think some of this has been talked about before, but I'm not finding any answers, just flames.
 

 I have an application that runs on an RCM3900. Over the years the size of the app has increased
little by little as features were added.
 

 Yesterday I made the latest change, programmed a module, and nothing. Program wouldn't run. I 
backed out the change and it runs. I add the change back in and remove a different feature. It runs.
 According to DC, the total code size without the new changes, and that runs, is 457939. When I download
the program, the RFU says it's sending 494520 bytes. 
 

 When I add the new feature back in, DC reports total code size of 463850 bytes. RFU says it's sending
500164 bytes. 
 

 First question, what makes up the difference between what DC reports and the RFU reports for size?
Is the extra 36k the RabbitBios and that it's not included in the total code size DC reports?
 

 Second question, since program memory is 512K, which is really 524288 bytes, why does the program quit
running when I get close to 500k bytes? Is this a compiler bug or is there something put there by DC?
 

 Note, I'm using DC 9.62, runtime checking disabled, and debug disabled.