Laser powder bed fusion (LPBF) has great potential for the fabricating complex geometries with improved functionality. In combination with nickel alloys such as Hastelloy X, parts produced with this technology find usage in high-temperature applications. Many studies have focused on the microstructure of Hastelloy X fabricated via LPBF technology, but most have been performed on simple cubic geometries. Considering the aforementioned applications, the parts are often characterized by fine features, like very thin-walled structured, often in the sub-1 mm range. However, little is known about the LPBF fabrication of such structures. The study presented in this paper shows that wall thickness plays an important role in microstructure formation. In situ temperature measurements and thermal simulations showed a difference in thermal history. Local heat accumulation in the vicinity of the scanned tracks was observed for the sample consisting of multiple tracks. Moreover, this effect was enhanced with the number of tracks, leading to different melt pool morphologies. Significantly, coarser solidification cells were found near the sample edges. In addition, larger grains oriented parallel to the build direction were observed for the samples consisting of 3 and more tracks, while for the thinner samples, a very fine microstructure with random orientation was found.