Surnames in Finland are of a recent, taxation-based origin. Most lines of evidence point to them being gradually adopted only from the 16th century onwards. And ordinary country folk didn’t use surnames until the 1800’s. The -nen suffix is originally an eastern Finnish one, and, as pointed out, originally a (positive) diminutive, but already in the 16th century records used interchangeably with the -son / -poika-suffixes. The -la-suffix as in ‘Hietala’ is a western Finnish one, and denotes the farm where the family was originally from. Hietala would be a farm on sandy soil.
In eastern Finland, the swidden-based economy of the 1800’s was mobile and the surnames followed the people as in today, whereas in Western Finland the fixed field-based economy meant that for taxation purposes, the farm mattered, not the people living in it. So, people got the name of the farm, no matter their true relations (of course most people living on the same farm would be related, nevertheless).
Plenty of Finnish surnames were made up of whole cloth only some 100 - 150 years ago. No rhyme or reason there, apart from fashion.
Of course, today the surnames have traveled all over, and a person’s surname most often tells nothing of where she or her family is from, in any meaningful sense.