Which data exactly? During the day, most of the data is being stored with the sampling rate of 2 minutes. The sampling rate during activities depends on the mode you selected in the settings of your Garmin device. Usually it is in the "smart recording mode", saving new data points only when really necessary (a bigger change of some of the data occurred), so it is not necessarily 1s. For that, you have to enable the "Every second recording" mode. The exported GPX file then indeed contains data for every second.
