new exhaust => low idle

[cpginkpt]

Got a new catalytic and muffler. The cat is a Walker from Rock auto, but my exhaust shop had to fabricate some pipe and said he ended up using a muffler from a Honda Civic.

The muffler pipe is definitely small and I can see the connection from the cat to the muffler is downsized (pic)

When I picked it up I notice it idles much lower - HOWEVER he also has not hooked back up the downstream O2 sensor yet since he got the wring sized flange and had to order..

So I am wondering - is the low idle from the missing O2 (we will put it in next week) OR is it perhaps from the downsized muffler pipe?

Thanks.

'88 4x4 5 sp