Lookup stage and maximum document size with comments collection

Hi,

With this pipeline I get “total size of lookup stage exceedes…”
{
$match:
{title: “The Express”}
},
{
lookup: { from: "comments", let: {"id": "_id"},
pipeline: [
{$match: {$expr: { movie_id: “$$id”}}},
{$sort: {date: -1}}
],
as: “comments”
}
}

However changing match stage of lookup pipeline to:
{$match: {$expr: { $eq: ["$movie_id", "$$id"]}}},

fixes the error.

Why? I would be grateful for an explanation.

Cheers