Magnum condoms are only minimally wider at the open end or “bead” but they have a distinct flare and are significantly wider toward the closed end and are widest just about at the point where the corona of most guys winds up. Most condoms of “regular” size are cylindrical and of uniform thickness throughout.
For example, the Trojan-ENZ condoms that come in the blue box have a uniform flattened, unstretched width of 52 mm throughout their length. Trojan Magnums that come in the black box have an unstretched flattened width of 54 mm at the bead, only 2 mm wider than the “regular” Trojans, but they flare out to a maximum width of 60 mm near the closed end (tip).
Now 54 mm flattened width equates to an unstretched circumference of only 4.25”, but condoms have to be stretched some to gain enough purchase not to slip off. A guy who is around 5.25” in girth at the base will stretch them about 25% at the bead, and that will probably be plenty to keep them on.
If a guy has a lot of girth at the base but a tree-trunk shape (like me) the Magnums aren’t really much better at relieving pressure at the bead. But if a guy has a baseball bat shape, or a very prominent glans they are easier to put on and more comfortable to wear. Also, some guys like the sensation of the tip of their cock sliding around in a somewhat baggy-ended condom.