I'm after a small(ish) rack top bag - i.e. one that sits atop the rear pannier rack.

The one that has caught my eye is the Topeak DXP Trunkbag:

https://www.topeak.com/global/en/pro...8-trunkbag-dxp

I like the fact that it can expand if required and has panniers if extra capacity if needed. However, most of the time it will remain nice and compact.

Are there any other similar bags that I should consider?

i have a bontrager that uses a quick release button to secure and remove it from the rack although it may be propietary to a bontrager rack which I have. My wife also has the bontrager trunk and same rack but they lbs didn't have the quick release in stock when she bought hers only the velcor straps she regrets getting the velcro one although it seems just as secured as mine. Both have expandable sides.

I have a rockbros copy of that trunk bag that I got for $30 shipped
https://www.ebay.com/itm/Rockbros-MT...sid=m570.l1313

It is very good quality but fits my rack okay. My rack is a somewhat narrow style - it would fit much better on a wider rack