If you want to get all instances of lines that have "217848" and "BIG" in the files, you have to use
grep -E "217848|BIG" <filename or list of files>
or
egrep "217848|BIG" <filename or list of files>
Using grep "217848" <file or list of files> | grep BIG, or grep "217848" <file list> |grep BIG will only give you one return, even if there are many lines that have both "217848" and BIG in them.