I have always liked Rabbi Dr. Tzvi Hersh Weinreb's reflections on the pslams on the OU site. The excerpt below is from:
http://www.ou.org/index.php/torah/article/psalms_chapter_18/search4
.....
Psalm 18 is very nearly the same as the text of II Samuel 22...
"The most noteworthy difference between this chapter and the way the Psalm appears there is the last verse, which is recited at the end of bentching (birkas hamazon, grace after meals). In Samuel it reads, "migdol yeshuos malko," that G-d is the tower of His chosen king's salvations.
Here, it says "magdil yeshuos malko," that G-d increases His chosen king's salvations. In his personal version of the Psalm, David used the word migdol - tower - referring to his own complete salvation. For the "public" version here in Psalms, David used magdil - increases - referring to gradual growth leading towards the ultimate salvation.
On weekdays, we bentch using magdil, referring to gradual growth. On Shabbos, which is a taste of the "World to Come," we bentch with the word migdol, reflecting the eventual completion of our salvation as a people. "
