[answer] Different icon sizes

[b1nhack]

This issue often occurs in Neovim, such as with lazy.nvim or nvim-cmp.

In lazy.nvim, the same icon has different sizes on different lines.

You can fix this issue by setting pseudo-transparency to 0.

vim.o.winblend = 0
vim.o.pumblend = 0

in nvim-cmp

window = {                  
	completion = {          
		winblend = 0,      
	},                      
	documentation = {       
		winblend = 0,      
	},                      
},                          

The default values for these options are 0, so most of the time you just need to remove them from the configuration file.

root cause

As explained in the Neovim documentation for the winblend option:
Enables pseudo-transparency for a floating window.
These transparency effects are all pseudo-transparency.
This means that floating windows with pseudo-transparency enabled render background characters at the same level. The background and foreground are not actually distinguished but are blended together.

However, Kitty determines the size of icons by following them with spaces. When pseudo-transparency is enabled, if there are background characters under the spaces following the icons, the spaces will blend with the characters, causing the spaces to disappear. This results in icons appearing in different sizes.