View Source Avrogen.Avro.Types.Logical.BigDecimal (avrogen v0.10.0)
This represents the logical type of the scalable precision decimal introduced in version 1.12.0
Like the fixed precision decimal logical type, it represents an arbitrary-precision signed decimal number of the form unscaled × 10-scale. However, the scale and precision do not need to be known in advance.
To quote the docs:
Here, as scale property is stored in value itself it needs more bytes than preceding decimal type, but it allows more flexibility.