Several reasons can lead to this:

The Kurzweils have a 64kb limit to the size of a single sequence.

You can check your sequence size by going to MASTER - OBJECT - NAME. Scroll down to your sequence name and it will tell how many bytes it contains. Remember your limit is approximately 65,000. And your machine may only be able to hold 2 of these size sequences if you do not have the PRAM upgrade.

Mono Pressure (Aftertouch) is typically enabled and recorded into the sequence, and depending on your playing style can make your sequences 2-10 times larger than what they need to be. (Each time you rest your fingers on the keys it records how much pressure you're putting on them as fast as it can). Here's how you disable it from being recorded in the next sequence you write:

Press MIDI - XMIT, and change Press: Mpress to OFF by clicking the data wheel left one notch.

And here is how you can delete it from the sequence you're working on now:

Go to your sequnce, press EDIT, TRACK, choose Funtion: ERASE. Press the CHAN/BANK buttons to select Channel: ALL in the top-right of the window. From: should be from the beginning to end measure of your sequence or end could be a just a large number. On Events, select MonoPress and hit GO (Done will not perform the operation). When finished, press Done and exit out, you will have the chance to abort this or save. Now go back to objects and see how much size was saved.