Hello Akshat,
I think the reason is because you didn't set the map extent when you load the mapview. The map is not taking the full height due to view is out of the level of detail (LOD).
The easy way to fix is to set a map extent, please take a look about this documentation about Navigating the map—ArcGIS Runtime SDK for iOS
Here I also shared with you a snippet about how to set a map extent under the (void)viewDidLoad function, also be careful the xmin, ymin, xmax and ymax should match with your tile map spatialReference:
// Add a basemap tiled layer
NSURL *url = [NSURL URLWithString:@"http://services.arcgisonline.com/arcgis/rest/services/ESRI_StreetMap_World_2D/MapServer"];
AGSTiledMapServiceLayer *tiledLayer = [AGSTiledMapServiceLayer tiledMapServiceLayerWithURL:url];
[self.mapView addMapLayer:tiledLayer withName:@"Basemap Tiled Layer"];
AGSEnvelope *envelope = [AGSEnvelope envelopeWithXmin:-124.83145667 ymin:30.49849464 xmax:-113.91375495 ymax:44.69150688 spatialReference:_mapView.spatialReference];
[self.mapView zoomToEnvelope:envelope animated:NO];
Hope this can help.
Best Regards,