I'm on Unity 6.x and I've noticed that the problem happens when I'm on some other platform than Standalone.

The fix is to switch to Standalone, then disable the lowres setting, and then switching back to whatever platform you were on.

More details:
https://discussions.unity.com/t/game-view-scale-on-compilation-play/217998/8

I'm still seeing this issue in a fresh project with fresh install of 6000.0.38f1. I have multiple monitors and my primary 4k one has 150% font size. My Game tab is popped out and displayed on my second 2k monitor with 100% font size. Every time I hit play the Game tab scale slider changes back to 1.5x.

I can temporarily fix it by closing the game tab and opening a fresh one, but I'm sure it will be back. I've noticed this problem since at least Unity 2022.

2022.3.44f1 not fixed. 1 monitor here is pastebin(com)/n7vkigHq

Ran into this problem after 'upgrading' from 2023.1.10f1 to 2023.2.20f1.
As a workaround, put this script in Assets/Editor/
----

using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
public static class GameViewScaleFix
{
static GameViewScaleFix()
{
// Subscribe to play mode state changes
EditorApplication.playModeStateChanged += OnPlayModeStateChanged;
}

private static void OnPlayModeStateChanged(PlayModeStateChange state)
{
// Check if we just entered play mode
if (state == PlayModeStateChange.EnteredPlayMode)
{
// Reset the Game view scale
ResetGameViewScale();
}
}

private static void ResetGameViewScale()
{
// Find the Game view window
var gameViewType = typeof(Editor).Assembly.GetType("UnityEditor.GameView");
var gameView = EditorWindow.GetWindow(gameViewType);

// Use reflection to get the private "m_ZoomArea" field in the Game view
var zoomAreaField = gameViewType.GetField("m_ZoomArea", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
var zoomArea = zoomAreaField.GetValue(gameView);

// Get the "scale" field of the zoom area (it's actually a field, not a property)
var scaleField = zoomArea.GetType().GetField("m_Scale", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);

// Set the scale to (1, 1) to reset the zoom
scaleField.SetValue(zoomArea, new Vector2(1f, 1f));

// Repaint the Game view window to apply the change
gameView.Repaint();
}
}

I have found a working workaround for this issue for anyone who has the same problem.

1. Right click on the "Game" tab at the top of the screen
2. Click "Close tab"
3. Right click on an open tab and select "Add tab > Game"
4. Change view from "Free aspect" to desired resolution
5. Change scale to desired value and hit play

Now the game will start at the proper scale every time.

The workaround doesn't work for me, as "Low-Resolution Aspect Ratios" is greyed out (my main, primary monitor, where my game window is, has scale set to 100%, which greys out and ticks "Low-Resolution Aspect Ratios"). My other monitor (has the console window on it) has scale set to 125%, which causes the 1.3x scale problem for me.