Its not the weight of the hammer, it’s the inherent magic. You can only lift it if you are worthy. Which is why only a handful of people have been able to lift it aside from Thor.
There’s a theory going around the hammer isn’t heavy as much as it’s stubborn. That’s why when Thor placed it on the coat rack it didn’t fall and destroy everything, and on the other hand the hammer could also pin down Loki easily (kudos to that post, couldn’t find it myself)
Mjolnir has been wielded by a select number of other individuals: alien Beta Ray Bill; Avenger Captain America;Eric Masterson;Odin (Thor’s father); Bor (Thor’s grandfather); and Buri (also known as Tiwaz, Thor’s great-grandfather).
The hammer has also been lifted by various sentient constructs (non-sentient machines apparently cannot), such as Zarrko the Tomorrow Man's mining robot; the Air-Walker (animated by the soul of Nova Corps captain Gabriel Lan); and the Awesome Android (by mimicking Thor’s abilities and worthy nature). The hammer has also been lifted by Earth itself when animated via magical means.
Several other characters from outside the primary continuity have lifted the hammer, including: Conan the Barbarian; Dargo Ktor (Future Thor); Loki;Magni;Rogue (after absorbing the entirety of Thor’s life force and thus essentially becoming Thor);Miguel O’Hara (Spider-Man 2099); Woden; Alex Power; and the DC Comics characters Superman (though he was unable to later that issue and Thor claimed Odin had briefly lifted the enchantment) and Wonder Woman.