Why aren't my system messages' FontSize changing?

Hello all, I have some code that is supposed to send a system message in the chat about who the player is and if they joined or left.

Now, the FontSize in the OnClientEvent in the LocalScript isn't changing. I know that the default value is 18, but I set it to Size12 on Line 13 and it didn't change the size.

Is this because of the parameters or the arguments or something?

LocalScript:

local StarterGui = game:GetService("StarterGui")
local Players = game:GetService("Players")
local Player = Players.LocalPlayer 
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage:WaitForChild("PlayerJoinedorRemoved") 

RemoteEvent.OnClientEvent:Connect(function(playerToDisplay,action) 
	if action == "Joined" then
		game.StarterGui:SetCore("ChatMakeSystemMessage", {  
			Text = ("[SYSTEM] "..playerToDisplay.Name.." has joined the server.");
			Color = Color3.fromRGB(97, 255, 142);
			Font = Enum.Font.SourceSansBold;
			FontSize = Enum.FontSize.Size12
		})

	elseif action == "Left" then
		game.StarterGui:SetCore("ChatMakeSystemMessage", {  
			Text = ("[SYSTEM] "..playerToDisplay.Name.." has left the server.");
			Color = Color3.fromRGB(255, 51, 71);
			Font = Enum.Font.SourceSansBold;
			FontSize = Enum.FontSize.Size12
		})
	end
end)

Server Script:

local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage:WaitForChild("PlayerJoinedorRemoved")

Players.PlayerAdded:Connect(function(playerAdded)
	RemoteEvent:FireAllClients(playerAdded,"Joined") 
end)

Players.PlayerRemoving:Connect(function(playerRemoved)
	RemoteEvent:FireAllClients(playerRemoved,"Left")
end)