Let's take a gnome for the example. Here is what the PHB 37 says for its size:
Size. Gnomes are between 3 and 4 feet tall and average about 40 pounds. Your size is small.
The PHB 176 also says the following for the carrying capacity:
Carrying Capacity. Your carrying capacity is your Strength score multiplied by 15. This is the weight (in pounds) that you can carry, which is high enough that most characters don't usually have to worry about it.
Size and Strength. Larger creatures can bear more weight, whereas Tiny creatures can carry less. For each size category above Medium, double the creature's carrying capacity and the amount it can push, drag or lift. For a Tiny creature, halve these weights.
If this gnome has a Strength of 10, it means it can carry 10*15=150 pounds ! More than the triple of its own weight !
Am I missing something or can small characters really carry that much ?
PS: I know D&D isn't meant to be a realistic simulation, but still.