Routing large RF trace to small SMD pad
You have several approaches you can use:
First if this a relative low frequency (below 500MHz) you can stay at 50ohms as close as possible and then narrow the line with narrow taper.
Second if the frequency is higher you can narrow the RF layer to the extent that the 50 ohms line will be narrow as the pad.
Third, you can also use grounded coplanar line and keep the 50ohms by narrowing the gap of the ground. 
If non of these options don't work for you, you can use more complex grounding that i can send you per your request.