Rigidbody2D is falling slowly when using velocity

I am making a super mario style game. I have attached a rigidbody2d to my player and a script to make him run. When you run while falling, the gravity slows down. It works exactly how I want it to until this happens. The scene is at a reasonable scale (1, 1, 1) and the gravity works fine.

Here's the script-

 //MarioMove.cs
 using UnityEngine;
 using System.Collections;
 
 public class MarioMove : MonoBehaviour {
     public Animator marioAnimator;
     public bool canJump;
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void LateUpdate () {
         if(Input.GetKey(KeyCode.RightArrow)) {
             transform.eulerAngles = new Vector3(0, 0, 0);
             GetComponent<Rigidbody2D>().velocity = new Vector2(5, 0);
         }
         if(Input.GetKeyDown(KeyCode.RightArrow)) {
             marioAnimator.SetBool("Run", true);
         }
         if(Input.GetKeyUp(KeyCode.RightArrow)) {
             GetComponent<Rigidbody2D>().velocity = Vector3.zero;
             marioAnimator.SetBool("Run", false);
         }
         if(Input.GetKey(KeyCode.LeftArrow)) {
             GetComponent<Rigidbody2D>().velocity = new Vector2(-5, 0);
             transform.eulerAngles = new Vector3(0, 180, 0);
         }
         if(Input.GetKeyDown(KeyCode.LeftArrow)) {
             marioAnimator.SetBool("Run", true);
         }
         if(Input.GetKeyUp(KeyCode.LeftArrow)) {
             GetComponent<Rigidbody2D>().velocity = Vector3.zero;
             marioAnimator.SetBool("Run", false);
         }
     }
     void Update () {
         if(Input.GetKeyDown(KeyCode.UpArrow)) {
             if (canJump == true) {
                 GetComponent<Rigidbody2D>().AddForce (transform.up * 300);
                 canJump = false;
             }
         }
     }
     void OnCollisionEnter2D(Collision2D coll) {
         if (coll.gameObject.tag == "Ground") {
             canJump = true;
         }
     } 
 }