Despite common belief, characters tripping are not caused by weight. Instead, they are daused by how you took care of your Tamagotchi (V1-3), or fortunes.
On the V1, V2, and V3, a character trips when they are a below-average adult character. (Kaerutchi and Androtchi are 2 examples.) On the other hand, bad care characters (like Whaletchi and Masktchi) use the closed-eye sprite instead of tripping. Any character that is of neutral care or better (Nyorotchi and Chohimetchi are examples) will smile frequently.
On the other hand, V4s and V4.5s do not trip due to that. Instead, fortunes determine whether or not they trip. The last fortune (or the person flexing) determines if they trip. 1 star means they will constantly trip, 2 means sometimes, and 3 means never. However, this is only temporary, and changes during the next fortune you get.