Essentially the same argument as stocks.

Did spot one interesting bit, which I figure I should look at out of casual interest - study the composition / methodology behind some of the corp / EM indexes.

In any case, it is important to note that most of the bond indices tracked by ETFs and index funds are not pure market-cap weighted. This is particularly the case in areas of the bond market such as corporate bonds and emerging market governments, which are riskier than developed governments.

Indices for these riskier areas typically apply liquidity filters and impose caps at single issuer level to avoid becoming too concentrated. These are safety measures designed to ensure diversification of risk while aiding passive bond fund managers in the task of replicating the performance of the index.
https://www.morningstar.co.uk/uk/new...cost.aspx?ut=2