Because the amount matters?
If we talk about the impact of migration the amount certainly plays into what if any impact occurs.
It really seems like you're stuck on a sort of rhetorical talking point phrase.
In other words, "mass" doesn't refer to an amount, but a lack of filtration/selection/etc. Note also that this isn't even an inherently bad thing--there's something noble about wanting America's doors open to everyone: "Give me your tired, your poor, Your huddled masses yearning to breathe free...".
Policies and the impact (something that should hopefully inform a policy) are more nuanced than just "mass migration" "more migration" and so on. Your understanding seems entirely disconnected from actual policy.
I can't possibly explain something that is going to be different from person to person or politician to politician, even more so if for you it only ends up being categorized in weird generalities that read like they're someone else's rhetoric skewed talking points / loaded phrases. I don't think you can expect to understand something if you just generalize in that way.
I'm asking "what is your argument?" What argument are you making?
> Policies and the impact (something that should hopefully inform a policy) are more nuanced than just "mass migration" "more migration" and so on.
Of course, and still we speak in shorthand all the time because regurgitating the complete nuance in every single Internet comment is untenable. If "left-wing folks generally support mass immigration" is inaccurate or incomplete, let's talk about that--what nuance am I overlooking?
I really haven't seen this with "left wing folks". That sounds more like some folks talking points they want you to buy into.
I think you might be mistaken. Any immigration /= "more mass immigration".