New powers over energy, transport and running the assembly will be outlined in a draft Wales Bill on Tuesday.
Monmouth MP David Davies said he was worried that devolution "is always a one-way street".
"No-one is talking about taking powers away from the Welsh government in areas where it is performing badly, such as health or education," he said.
The draft bill will outline a "reserved powers" model which lists all the policy areas where Westminster remains responsible, with the rest assumed to be devolved.
Welsh Secretary Stephen Crabb has denied claims that the bill could weaken the assembly, and rejected calls by First Minister Carwyn Jones to delay publication for further cross-party talks.
Wales is edging closer to independence, a senior Welsh Tory MP has claimed, amid plans for further devolution.