I'm unsure where you get the idea that Frisians specifically would be massive.
The Dutch official statistics agency CBS (Centraal Bureau voor de Statistiek) has stats on the height of people.
For 2013, CBS gives the following average height of men and women:
| Gender | Heigth |
|---|---|
| Men | 180.8 cm |
| Women | 167.5 cm |
| Average | 174.1 cm |
Unfortunately this data is not split by province, so we cannot look at Frisians directly. They do have data on degree of urbanisation though, and since Friesland has quite a low degree of urbanisation compared to for example Holland and Brabant, we can use that as a proxy. Here is the data:
| Urbanisation | Heigth |
|---|---|
| Very high | 173.5 cm |
| High | 174.1 cm |
| Moderate | 174.3 cm |
| Low | 174.3 cm |
| Very low | 174.9 cm |
Most Frisians will live somewhere with 'Moderate' or lower urbanisation, so on average they'll be slightly (0.2-0.8 cm) longer than an average Dutch person. I don't think that difference is large enough to call them "massive".