Your base squad size is 6. Each squad size technology increments this by an ascending multiple of three. Thus squad size 1 is 6 + 3, squad size two is 6 + 3 + 3x2, and squad size three is 6 + 3 + 3x2 + 3x3; or 9, 15, and 24. You can view your squad size from the "army camp" tab of your barracks. Tournaments use a percentage of your squad size for fights, though there is likely a minimum value of 6 or so. Since you are in chapter 1 you probably are seeing this tournament squad size pop up with the minimum value, and are assuming that your normal squad size has somehow decreased. Once you get a large enough squad size, the tournament squad size used will be 5% of your squad size times the tournament province number. That is the first tournament province would use 5%, the second 10%, etc.
EDIT: It appears that the basic (or starting) squad size did change at least a few times over the game's duration. According to the wiki, the basic squad size was 12, according to another source, it hit 15, and now is at 6. I do not think that the last change was recent enough to have been seen by you, as Mykan's fighting guide, last edited a year ago, does have the six value.