It could be that the Western Whiplash is a bit heavier, I did not weigh my Western Whiplash as it is also at my office. Could also be my scale is being wonky - I used my own regular scale and weighed myself without bag, and then holding bag, so it could throw things off!
You will be okay if you really only carry (a smaller) wallet, small makeup pouch, but it might be a snug or impossible fit when you start adding clothing items. I think you might be surprised at how small the Rogue 25 actually is in person (even I was, I mean my first comment when I opened the box was "that is so
adorable" which is a comment I usually reserve for tiny kittens). I don't have a Mercer 30, but it is supposed to be 2" longer, 1 1/4" taller, and 1/2" wider so I expect it does have more room in the middle compartment as well.
The dimensions of the bag don't seem that small online, but because of how it is separated, there is not a lot of room in the middle. My other smaller bags of similar or even smaller external dimensions that are just one compartment have a lot more room in them.
I have my Rogue 25 at work so I switched some stuff out of the KS Small Hayden I am wearing today just to take some quick and somewhat blurry photos (sorry, I have to use my front camera on my iphone and it is not easy to do!). Here is inside of Rogue 25 with a mid-sized wallet (Kate Spade Stacy), small soft cosmetic pouch, keys (in a soft pouch), and my sunglass case. I have to play some Tetris to get stuff out. I can't fit a cardigan in there, though I could stuff a very small/thinner silk scarf in if I was not concerned about being too careful - I would worry about it getting caught in zipper. I am using phone to take photos, but I would probably put it in outside pocket anyway.
View attachment 3769462
Here is photo with sunglass case removed, as well. No Tetris needed now at least!
View attachment 3769463
And with a thin cardigan complete with a bonus cat hair or two stuffed in (same stuff in it before as last pic, no sunglass case). I can't zip it up and phone would be a tight fit....looks like if I put it in side pockets it would stretch the leather on side (at corners of phone). I did not want to force it.
View attachment 3769464
View attachment 3769465