It seems that way, since those instruments are played in large concert halls that a guitar would probably need an amplifier for.
And if it's true, what is the reason?
“Pizzicato” may be performed on guitars as well as the other string members. The technique on both instruments exists in a dynamic curve and can be played any volume from “ppp” to snap pizz.
Violins might project more clearly due to a couple reasons: modern strings are typically steel or nickel-wound, holding more tension and producing a brighter, louder sound. Classical guitars on the other hand use nylon strings which are lower tension.
The other reason violin pizz might be more easily heard is due to register. The two instruments in question only overlap for a portion of their respective ranges. Pizz at lower and extreme upper frequencies aren’t as successful. More to say here but it falls outside the question so I’ll omit.
So, it’s difficult to say definitively as there are so many factors involved for sound production, but above are two reasons why a violin might have a “louder” pizzicato.