I don't know how long ago you "were a young lad" but i'm gonna guess that you're probably older than me, so the MTB you so fondly remember was probably a '26er' (559 wheel) MTB's went through a lot of changes in the 2000-2010's and comparing old bikes to modern ones is like apples-to-oranges.

Most modern MTB's are either 27.5 or 29 inch (650B or 700) As you might guess by the names, 27.5s are slightly bigger than 26", 29ers are very much bigger.
Having a bigger wheel, means you don't need as big of a chain ring to get the same mechanical progression (ie, how far the bike goes for one turn of the crank.

Throwing some rough numbers from the Cannondale Trail 8 into BikeCalc.com's Gear Calculator returns roughly 89 gear-inches for it's 36t-11t top gear. This compares pretty favorably to the 91 that my vintage 26" Cannondale F-1000 gets with it's 42t-12t
A hard-core 29er, like Cannondale's Scalplel 1 brings back a more-racy 92, with a 32t-10t top gear.

Fitting a 46t to a the Trail would return a really stiff 113-inch gear, which is about equivalent to the top gear on a lot of road-racing bikes; but they don't have 2.1" wide knobby tires to haul around.

Big, tall gears are great for scooping up the miles, but they tend to make a bike a lot more difficult to deal with for anything but cranking away on the open road