After choosing an enemy to fight, the 2nd one is always random. At level 1, spending a point or 2 in HP should help keep Sonny alive against non-food foes.
Outside of battle, you can right-click to open the main menu. There, next to Sonny's name, you can see his current weight.
In zones with NPC interactions, you can also see NPC's weight in this menu.
Each character's hunger is affected by their level. NPCs also automatically scale to Sonny's level.
Through the inventory, burgers from the restaurant shop can be used on characters to directly increase weight. Also, when you start a new day, units gain weight based on how filled their hunger was.
At 140lbs , 220, 420 and 640, units will go up a weight stage