With this question Jephthah is not asking whether the king was better than Balak in the way of strength or morals. Rather, he was asking whether the king had greater rights than the king of Moab. This is confirmed by his follow up question, Did he ever contend against Israel?
1
25 Now are you any better than Balak the son of Zippor, king of Moab? Did he ever contend against Israel, or did he ever go to war with them?