Group by size category as separate array in laravel php

I have on table size_chart

| id | category | size | created_at          | updated_at | deleted_at |
+----+----------+------+---------------------+------------+------------+
|  1 | Regular  | S    | 2017-10-11 09:35:14 | NULL       | NULL       |
|  2 | Regular  | XS   | 2017-10-11 09:39:34 | NULL       | NULL       |
|  3 | Regular  | M    | 2017-10-11 09:39:37 | NULL       | NULL       |
|  4 | Regular  | xxl  | 2017-10-11 13:03:52 | NULL       | NULL       |
|  5 | Regular  | l    | 2017-10-11 13:03:59 | NULL       | NULL       |
|  6 | Regular  | xm   | 2017-10-11 13:04:03 | NULL       | NULL       |
|  7 | Small    | em   | 2017-10-11 13:05:04 | NULL       | NULL       |
|  8 | standard | em   | 2017-10-13 07:16:50 | NULL       | NULL       |

In this table i have some sample data. I need to group the size based on category For.eg

Regular = [s,xs,M,l,xm]
standard = [em]
small = [em]

How to achieve this on my php controller function

public function sizeFetch(Request $request){
        $cat_id = $request->id;
        $size_id = SizeCategory::where('category_id',$cat_id)->pluck('size_id');
    $size_details = SizeChart::whereIn('id',$size_id)->get();

        return $size_details;
    }

Now this function return like this

[
{
id: 1,
category: "Regular",
size: "S",
created_at: "2017-10-11 09:35:14",
updated_at: null,
deleted_at: null
},
{
id: 2,
category: "Regular",
size: "XS",
created_at: "2017-10-11 09:39:34",
updated_at: null,
deleted_at: null
},
{
id: 3,
category: "Regular",
size: "M",
created_at: "2017-10-11 09:39:37",
updated_at: null,
deleted_at: null
},
{
id: 7,
category: "Small",
size: "em",
created_at: "2017-10-11 13:05:04",
updated_at: null,
deleted_at: null
}
]

Please help me to resolve this

via Chebli Mohamed