"天" seems bolder than other characters

[Firestar-Reimu]

"天" seems bolder than other characters here

Using Source Han Sans CN Regular 2.004 on Windows Notepad 11.2410.21.0

This issue also affects other simple characters like 大、小、本、干、不、力

[tamcy]

Due to the interpolation nature of the font, strokes may not look completely even between glyphs.
But what you are seeing is probably due to the ClearType rasterizer of Windows, as the horizontal strokes of 天 and 文 are virtually the same at your chosen size, but they look significantly different. Here's how the text looks like in Microsoft Word:

[Firestar-Reimu]

So this is due to Windows Notepad itself and not the font?

Can the font do something with it? for example act different on Windows or Mac/Linux?

[tamcy]

You may try to use the TTF varaint of the font built by third parties, like this or this. On Windows the TTF version may yield better result as Microsoft probably invested more on its TTF rasterization engine.