On the distribution database, you can execute the statement below to get the sizes of the internal replication specific tables
USE distribution
GO
SELECT Getdate() AS CaptureTime,
Object_name(t.object_id) AS TableName,
st.row_count,
s.NAME
FROM sys.dm_db_partition_stats st WITH (nolock)
INNER JOIN sys.tables t WITH (nolock)
ON st.object_id = t.object_id
INNER JOIN sys.schemas s WITH (nolock)
ON t.schema_id = s.schema_id
WHERE index_id < 2
AND Object_name(t.object_id)
IN ('MSsubscriptions',
'MSdistribution_history',
'MSrepl_commands',
'MSrepl_transactions')
ORDER BY st.row_count DESC