1. Tiles don't have to fit in the grid

If you want to reduce the number of Game Objects in the Scene at a time you can have a single symbol in your level definition represent a Game Object that spans multiple grid tiles. So if you want a lot of platforms that are 3 grid squares wide, you don't need 3 objects per platform, you can just use a single character to represent a 3x1 rect:

import kaboom from "kaboom"

// initialize context
kaboom()

// load assets
loadSprite("bean", "sprites/bean.png")

addLevel(
  // Note: the hyphens here hare just place holders to remind us that the
  // game objects created by ➁ and ➂ are actually taking up 2 and 3 grid
  // squares respectively.
  [
    "    ⚥                           ",
    "                                ",
    "            ➂--                 ",
    "                                ",
    "       ➁-                       ",
    "                                ",
    "                                ",
    "################################",
  ],
  {
    width: 32,
    height: 32,
    "⚥": () => [
        sprite("bean"),
        area(),
      body(),
      "player"
    ],
    "#": () => [
      rect(32, 32),
      outline(2),
      area(),
      solid(),
    ],
    "➁": () => [
      rect(32 * 2, 32),
      outline(2),
      area(),
      solid(),
    ],
    "➂": () => [
      rect(32 * 3, 32),
      outline(2),
      area(),
      solid(),
    ],
  }
);

const player = get("player")[0];

player.onUpdate(() => {
  const left = isKeyDown('left') || isKeyDown('a');
  const right = isKeyDown('right') || isKeyDown('d');
  if (left && !right) {
    player.move(-500, 0);
  } else if (right && !left) {
    player.move(500, 0);
  }
  camPos(player.pos);
});

onKeyPress("space", () => {
  if (player.isGrounded()) {
    player.jump();
  }
});

Obviously if you had many different shapes and sizes this would be quite onerous.

2. Advanced: Quad Trees and Loading/Unloading Regions

I actually ran into this problem myself on a recent Kaboom project and decided to completely overhaul the built in addLevel() with my own implementation that loaded from a bitmap instead of a bunch of strings, and then organized the level data into a quadtree so that I could quickly find chunks that overlapped the visible area, and load and unload game objects based on their visibility. The technique and code are a bit to complex it include here, so I'll just link to the Repl and the relevant source code: level-loader.ts and lib-quad-tree.ts and the usage in level-one.js .