When Artim fell and Data picked him up he looked like he was struggling carrying him. Afterwards, Data picked up a large boulder with no problem (like it wasn't heavy).
Why was Artim heavier to carry than the boulder?
The very short answer is that in-universe, Data had no (evident) problems with Artim's weight. Picard passed him over and Data carried him a short way and then put him down. Since we know that Data can effortlessly carry a boy that size, any imagined exertion is simply down to your own incorrect perception.
Picard reaches out for her... just as the whole thing comes down on both of them... Picard throws Artim out of the cave just in front of the collapse into the arms of...
who pulls him safely away from the collapse...
Abruptly, the boy stumbled through the opening, and Data’s strong arms caught Artim’s thin ones.
With astonishing ease, the android pulled the boy away from the danger.
At the sight of Data’s steady, placid expression, Artim felt a surge of relief-, that relief vanished at once when the boy turned to see the devastation he had left behind.
Out-of-universe, this is a perfect example of Bellisario's Maxim;
"Don't examine this too closely"
Although the character of Data has superhuman strength, what you're actually looking at in this scene is actor Brent Spiner (a relatively slight man who's just turned 50) carrying a ten year old boy weighing approximately 30-35 kilos. In the next scene he's pretending to lift heavy boulders which are in fact polystyrene balls sprayed grey by a guy named Clete.