The omniheat lining provides super insulation with minimal bulk any the vest is virtually weightless.  Much, much less bulky than my down filled vest.

The sizing is surprisingly slim (for Columbia).  At 5'11" with 33" waist and 41" chest.  I am basically a universal size medium.  I have many Columbia products and normally I find they are either true to size or slightly large.  All my other Columbia tops are medium.  This is the first time I found one of their products to be too small in a medium.  I have a fleece Columbia vest in medium and it is 2 inches wider at the chest and waist compared to this one in medium.  I saw a few reviews at Columbia.com that said this is true to size but I disagree.  At least the medium runs small.  I exchanged for the large.

Medium measures 40" at chest and 38" at bottom, 28" long.

Large measures 44" at chest, 42" at bottom, 29" long.

Also, there is absolutely no stretch in this vest so don't expect it to fit snug and "give" a little.

If you plan to wear this over only a single thin layer than your normal size would probably work.  I typically expect to wear a vest over at least a couple heavier layers.  So going up one size was necessary for me.  Consider that I usually wear my medium Columbia fleece vest over a shirt plus sweater or sweatshirt and it still has room to move.

This vest is supposed to work with the "Zip-in Interchange System".  I don't have any compatible shell to test this vest with.  Perhaps you need to match medium to medium, or large to large when using this as a liner with an interchange shell.  I do not know.

Overall this works great for me.