There not wide, they just have longer arms.
I recall reading somewhere an article about the different body types in AD&D. Compared to typical human ratios, dwarves' arms were only 85 to 90% as long, halfings were 90 to 95%, elves were at 100 to 105% and gnomes were 105 to 110% So...if a typical 6 foot tall human has a 6 foot armspan, a typical 4 foot tall dwarf would have a 3'6" to 3'9" armspan and so would a typical 3 foot tall gnome.
Or as Lisa Simpson said, "Like, you know....whatever."
