Using the follwing mapping will swap precision and scale:
{{
<map name="decimals">
<key column="id" />
<map-key type="int" column="idx" />
<element type="big_decimal" column="value" precision="28" scale="16" />
</map>
}}

Example for schema update:
create table xxxx (id integer, idx integer, value decimal(16,28), primary key (id, idx))

This is caused by a bug in org.hibernate.boot.model.source.internal.hbm.PluralAttributeElementSourceBasicImpl that send precision as scale and vice versa to Helper.interpretSizeSource.