The entire large file is encoded into the bank. This is to permit specific forms of processing that might require the entire length of the file, such as looping and pitch adjustment.
If you want to only build the displayed section of the file into the bank, you can right-click on the instrument and select “Replace Audio with Trimmed Copy” from the context menu. This creates a copy of the audio file that contains only the sampledata displayed on the instrument’s trigger region, and sets the instrument to use the new file instead of the original.
“Replace Audio with Trimmed Copy” does not alter the original file, or any other instruments that use that file; the trimmed audio file appears in the assets browser alongside the original version, and other instruments continue to use the original file instead of the trimmed copy.