It is a way to read the Gospels, although it could be argued that it's not what they say literatim. However, it's not particularly medieval, but has been around since early Christianity to our days. Take e.g., NVG Mt 7:13-14:
Intrate per angustam portam, quia lata porta et spatiosa via, quae ducit ad perditionem, et multi sunt, qui intrant per eam; quam angusta porta et arta via, quae ducit ad vitam, et pauci sunt, qui inveniunt eam!
The Contemporary English Version of the Bible translates this as:
Enter through the narrow gate; for the gate is wide and the road is easy that leads to destruction, and there are many who take it. 14 For the gate is narrow and the road is hard that leads to life, and there are few who find it.
Christian teaching has always seen life here as meaning eternal life, Heaven, and destruction (actually perdition, ruin,) as hell (or at least not-Heaven in more lax interpretations.)
Jesus also gave a hint of this at other times, at the point that the disciples ask:
"Quis ergo poterit salvus esse?". Aspiciens autem Iesus dixit illis: "Apud homines hoc impossibile est, apud Deum autem omnia possibilia sunt". (NVG Mt. 19:25-26)
"How can anyone ever be saved?" Jesus looked straight at them and said, "There are some things that people cannot do, but God can do anything." (CEV)
Hence your quote, salvandorum paucitas, damnadorum multitudo, can be seen as a condensed form of a way to read that aspect of the Gospels. I believe the important part of the message is not how many go into each group, but that Jesus taught the way, and that although it's impossible for the strength of man alone to achieve it, with the help of God anyone can. But we have to let God help us, and confront our pride and self-reliance (which are in fact very common.)
Unfortunately, the only results I get when looking for the full sentence are the lyrics of a heavy-metal band's song, and a book from the XVIII century whose contents I can't reach. If you could provide a link to these Latin documents you mention, it could help too.