So a bitmap index requires a bit per unique value IIRC (plus the key and some amount of overhead). So for ~32 unique values you're already at 4 bytes, 40 bytes per key-value pair for 320 values, etc.
In comparison, a B-field will let you store 32 distinct values at ~3.4 bytes (27 bits) per key-value pair at a 0.1% FP rate.
In comparison, a B-field will let you store 32 distinct values at ~3.4 bytes (27 bits) per key-value pair at a 0.1% FP rate.