Are the two fonts from the same foundry?

Some of the italic letters, like the M, do look wider.
But the metrics -- character widths (including sidebearings) and heights -- are the same.

The appearance of wideness is due to the fact that for a rhombus and rectangle of the same height and base, the rhombus will have longer sides. Note that the stroke weights of problematic characters, such as M italic, are slightly less than those of the roman, to compensate for this effect.

The naming scheme is really two systems, alphabetic and numeric.
So in long form it would read: "Helvetica 45: Light Roman" and Helvetica 46: Light Italic".

The numeric system is modern European (after Univers), but the more traditional method was felt necessary for the fuddy-duddy numbskulls in North America and elsewhere, just to confuse us with both.
If you think that's bad, try buying timber in Canada, where one has to contend with metric and imperial measurements, as well as real and nominal size!

Thanks for the replies and the technical answer,

I noticed that it's the Linotype version of the font that is creating this problem (for both the Light and Italic versions). If I instead use the Adobe Systems version of the font, ie. for both the Light and Italic versions, I don't get this problem. So it seems to be vendor-based.