I was at the Indian Wells Tennis Garden for the BNP Paribas Open and took the following photo of center court.
It is not an optical illusion – some courts have a thicker baseline painted because it is supposed to
1) make it a little easier for the people calling the lines to see whether the ball was in or out. Not all courts have a thicker baseline, but it is more common among courts used for professional matches. Thus, your standard public park / recreational courts are not likely to have a thicker baseline.
2) It typically helps the players see the baseline opposite them a little easier – since it is the line farthest from the players viewing position.
Also – it is mentioned in the specifications for building a tennis court. See Page 9 here:
2.5.1 Court Lines.
The centre service line and centre mark line must be 50mm wide. All other lines except the base line may be 25mm-50mm wide. The base line may be 25mm-100mm wide.
Personally I have played on courts with both a standard thickness base line and a 100mm thick baseline and it has not made much difference to me as far as visibility is concerned.
Here is an additional page that is a good reference on court dimensions and line widths.