MongoDB Projection on nested collection with csharp driver in .NET

Question

In the near future I want to migrate from SQL Server to MongoDB. For now I'm just playing around with all the possibilities and testing out the best schema design. I stumbled on a question. Let me summarize the issue.

I have 30 manufacturers that each contains a list of size ranges (like S -> XXL). Each size range has list of sizes (like S, M, L, XL, XXL). So this is the class I've created:

public class Manufacturer : BaseEntity
{
    #region Ctor

    public Manufacturer()
    {
        SizeRanges = new List<SizeRange>();           
    }

    #endregion

    #region Properties    

    public string Name { get; set; }
    public string Description { get; set; }
    public string MetaTitle { get; set; }
    public string MetaDescription { get; set; }
    public double PurchaseDiscountPercentage { get; set; }
    public bool LimitedToStores { get; set; }
    public int DisplayOrder { get; set; }
    public State State { get; set; }
    public bool SyncToShop { get; set; }
    public bool Deleted { get; set; }
    public bool ManufactuerTierPriceHasChanged { get; set; }
    public bool ManufactuerSizesHasChanged { get; set; }
    public bool PurchaseDiscountPercentageChanged { get; set; }
    public DateTime? DateChanged { get; set; }
    public DateTime? DateCreated { get; set; }

    #endregion

    #region Mapping

    public Picture Picture { get; set; }
    public List<SizeRange> SizeRanges { get; set; }       

    #endregion

    #region Classes

    public class SizeRange
    {
        // Ctor

        public SizeRange()
        {
            Sizes = new List<Size>();
        }

        // Properties      

        public string Name { get; set; }
        public int DisplayOrder { get; set; }
        public bool Deleted { get; set; }
        public DateTime? DateChanged { get; set; }
        public DateTime? DateCreated { get; set; }

        // Mapping

        public List<Size> Sizes { get; set; }
    }

    public class Size
    {
        // Properties           

        public string SizeName { get; set; }
        public string LookupSize { get; set; }
        public int DisplayOrder { get; set; }
        public bool Deleted { get; set; }
        public DateTime? DateChanged { get; set; }
        public DateTime? DateCreated { get; set; }
    }      

    #endregion
}

Now here is my question. What is the best way to get all the size ranges? I wrote this piece of code using mongoDB 3.2. I'm using 2 steps here but think there is a better way to get all the size ranges from all the manufacturers. If there is a better way, just let me know.

var sizeRanges = await erpContext
            .Manufacturers
            .Find(FilterDefinition<MongoManufacturer>.Empty)
            .Project(x=> x.SizeRanges)
            .ToListAsync(); // results in a list containing 30 collections of size ranges

var sizeRangesList = sizeRanges
            .SelectMany(x => x)
            .ToList(); // get a list with only the size ranges.