There's interesting neuroscience behind this. The brain maintains a "body schema" — a neural representation in the posterior parietal cortex that encodes body dimensions and spatial boundaries.1 This schema is updated through sensory feedback (proprioception, touch, vision) but it has significant temporal lag, especially after rapid changes.
Studies on phantom limb patients show the same principle — the brain's body map persists even when the physical reality has changed. After significant weight loss, the old body schema can persist for 6-24 months.2
One study found that individuals who lost >50 lbs still overestimated their body width by an average of 17% six months post-loss.3
The brain WILL recalibrate. But it's a gradual, neuroplastic process, not a switch.
1 Longo & Haggard, "An implicit body representation underlying human position sense," PNAS, 2010.
2 Ramachandran & Hirstein, "The perception of phantom limbs," Brain, 1998.
3 Chalklin et al., "Body size estimation following weight loss," Body Image, 2019.