Why a smaller, rounder stone changes the mix
Larger crushed stone is angular and interlocks with itself, which is part of what gives a standard mix its bulk strength economically, since less cement paste is needed to fill the gaps between big, jagged pieces. Pea gravel is small and rounded, so it packs differently, needs more paste to coat and bind it, and moves more easily through tight forms, narrow reinforcement or small diameter line, which is the whole reason anyone reaches for it in the first place.
That easier movement is also why it shows up under exposed aggregate finishes and in forms too tight or too congested with steel for a standard mix to work into cleanly.
Where the tradeoff gets missed
The paste that pea gravel needs more of is the more expensive part of the mix, and a slab built with more paste and less structural stone shrinks more on its way to setting than one built with the bulk aggregate it is standing in for. Used across a job that does not need it, a pea gravel mix costs more per yard and gives back nothing the job was asking for. It earns its place in the specific spots where a standard mix genuinely cannot get in or cannot finish the way the job calls for, not as a default upgrade.
The reverse mistake shows up on the pump side. Small, rounded aggregate travels line more easily than large crushed stone does, but grading across the full range of sizes still matters more than the top size alone, and a mix with a gap in that spread can separate in the hose even with small stone in it. Telling the plant the load is pumped, and through which line, still belongs on the order regardless of which aggregate is going in.