Just a thought, only 6 of the original 29 of Wu Qi bingfa remains to this day. If it was not lost, do you think it would have been more popular today than the Sunzi Bingfa considering it only contained 13 chapters ?

That's depend on the contents, which we'd never know until someone rediscovers them.

But from observations, volume is not a measure of how popular something will be.

Conciseness actually helps more here 'cos people just can't remember too many things at one go.

That's why Stephen Covey limited it to 7 Habits, not 13 or 39 even though he could have done it that way if he wanted to. Note the number of supplements he published as add-ons later, after 7 Habits become established.

Sun Zi's 13 chapters were quite comprehensive as an intro to military warfare.

How about Jiang Tai Gong 6 Strategies?

Sun Tzu is remembered because his treatis is the oldest complete version in Military History. Wu Qi was approximately his time. My belief is that both will have their own following. Divided into a Wu Qi camp and a Sun Tzu camp, disciples will argue over which was the better strategist, and which one is more essential to battle. Military schools will study both, and each can compliment and contrast with each other. However, I haven't read Wu Qi's Art of War, so I don't know the quality of Wu Qi's writings.