Fitbit Inc. surged in its trading debut, after heavy demand for the company’s initial public offering led it to boost the size of the deal. The San Francisco-based company’s appeal lies in its strong position in the fast growing market for wearable technology, in this case fitness trackers that can monitor everything from heart rate to sleep patterns. Fitbit is profitable — making it a standout among recent technology IPOs — with sales that nearly tripled last year.