Yes, I think you are missing something.
Please take a look at Strategy.Position and Strategy.Portfolio objects in OpenQuant.API. These objects offer access to strategy portfolio composition and history, as well as to a list of properties and methods usually used in position sizing / risk allocation (GetCoreEquity, GetTotalEquity, GetPositionValue, GetAccountValue, GetMargin, etc.)
You can backtest and trade equities, futures, options, ETF, Forex and potentially all other instruments in OpenQuant since its core framework is based on FIX (Finanacial Information eXchange) 4.4. standard.