Size really depends on a few things while this would be the average size of an hatchling out of the egg it can also be the size of a snake that was very small upon hatching, some will hatch as low as 40 grams some even smaller but it's rare, and then it also depends on how fast the animal get started, some will eat right after their first shed at 10 to 14 days some may take 6 to 8 weeks to get started.
Good news is that it can be fixed, if the animal has a good feeding response feed a hopper every 5 days and upgrade has it grows.
Considering your snake is a BEL but the head is cleaner than a Super Mojave it could be a Hypo Super Mojave and if there is spider in it it would definitely explain the backwardness during feeding, as wobbling is something any mutation with spider in it will do to a degree (from severe to mild)