Is the SP101 really that big?

Hah. It's all relative, John. BTW, the barrel lengths are figured differently. The M&P length includes the chamber, the revolver barrel length does not.

 

While the SP101 isn't an N frame it also isn't all that small or lightweight either. 27 oz. empty. IMO the M&P will conceal easier than the SP101 in 3". I carried OWB the SP101 327 for a short time. Compared to carrying an SR40c the Sp101 was harder to conceal due to the barrel of the SP101 would sometimes be poking out below a jacket.
S&W M&P specs
Specification

M&P9 Glock17
Capacity 17+1 17+1
Barrel Length 4.25” 4.49”
Trigger Pull 6.5lb 5.5lb
Overall length 7.63” 7.32”
Weight (empty) 24oz 22.04oz
Overall height 5.5” 5.43”
Width 1.2” 1.18”
Sight Radius 6.4” 6.49”
...............
SP101


Weight:
27.0–28.0 oz (765–794 g)

Length:
8.0″ (203 mm)

Barrel Length:
3.1″ (78 mm)

 

That's a "real" gun on the left.....

 

Yes it's that big,I have a SP 101 .357 Magnum 3"

 

Those are some mighty fine looking grips right there!

 

The other side is where the grip got dinged when recoil tore the gun from ifithitu's hand.

 

Lol ifithitu,have some strong man hands!

 

Bigger than a LCR, but smaller than a Speed Six.

17 ounces is all I want to carry on my ankle.

 

I don't really like carrying on my ankle. Maybe an LCP wouldn't be so much but even an LCR isn't that comfortable in an ankle holster. By the time I got my pant leg up the BG would already have shot me.