Apophis is the main antagonist of SG-1 for the first couple seasons. In The Serpent's Lair SG-1 destroys 2 Ha'tak vessels right before they attack earth. This is shown as breaking Apophis' hold on much of his territory, and even the people on what seem to be his capital world (Chulak) start to rebel more openly. How could he have maintained his position as a System Lord if losing 2 ships in battle completely broke him? We see much larger fleets later, so why were those 2 ships so key to his forces that he was utterly defeated less than 1 year later (barring when he assumes command of Sokar's much larger fleet)?
So very simple yet oh so difficult to articulate an explanation.
Goa'uld society is not very well explained beyond that it is a feudal society and very Machiavellian.
Each system lord has to have many underlings that manage territory and resources for them and each underling wants to move up. So when Apophis has two ships destroyed, some of his underlings defect, meaning he loses access to the 6 ships they controlled. This weakens him further, so a minor Goa'uld attacks him and takes a planet, and so on and so forth until he is defeated and captured by Sokar.
On a somewhat separate note: while we do see much larger fleets later, this is partly because the Tau'ri are royally screwing with the Goa'uld status quo.